diff --git a/docs/private b/docs/private
index 6523cdc8..1840731c 160000
--- a/docs/private
+++ b/docs/private
@@ -1 +1 @@
-Subproject commit 6523cdc838a302cbbad413c14128c6fb312f7508
+Subproject commit 1840731c685509f4874650c1454ba621b9987d76
diff --git a/src/mir/join_ir/json.rs b/src/mir/join_ir/json.rs
new file mode 100644
index 00000000..4fc64f6b
--- /dev/null
+++ b/src/mir/join_ir/json.rs
@@ -0,0 +1,398 @@
+//! JoinIR JSON シリアライザ (jsonir v0)
+//!
+//! JoinModule を JSON 形式でシリアライズする。
+//! 用途: デバッグ、テスト、将来の selfhost JoinIR ロワーの参照フォーマット。
+//!
+//! 仕様: docs/private/roadmap2/phases/phase-30-final-joinir-world/joinir_json.md
+
+use std::io::Write;
+
+use super::{
+    BinOpKind, CompareOp, ConstValue, JoinFunction, JoinInst, JoinModule, MirLikeInst, VarId,
+};
+
+/// JoinModule を JSON としてシリアライズする
+///
+/// # Example
+/// ```ignore
+/// let mut output = Vec::new();
+/// write_join_module_as_json(&module, &mut output)?;
+/// let json_str = String::from_utf8(output)?;
+/// ```
+pub fn write_join_module_as_json<W: Write>(module: &JoinModule, out: &mut W) -> std::io::Result<()> {
+    write!(out, "{{")?;
+    write!(out, "\"version\":0")?;
+
+    // entry
+    match module.entry {
+        Some(entry_id) => write!(out, ",\"entry\":{}", entry_id.0)?,
+        None => write!(out, ",\"entry\":null")?,
+    }
+
+    // functions
+    write!(out, ",\"functions\":[")?;
+    let mut first_func = true;
+    for func in module.functions.values() {
+        if !first_func {
+            write!(out, ",")?;
+        }
+        first_func = false;
+        write_function(func, out)?;
+    }
+    write!(out, "]")?;
+
+    write!(out, "}}")?;
+    Ok(())
+}
+
+fn write_function<W: Write>(func: &JoinFunction, out: &mut W) -> std::io::Result<()> {
+    write!(out, "{{")?;
+    write!(out, "\"id\":{}", func.id.0)?;
+    write!(out, ",\"name\":\"{}\""  , escape_json_string(&func.name))?;
+
+    // params
+    write!(out, ",\"params\":[")?;
+    for (i, param) in func.params.iter().enumerate() {
+        if i > 0 {
+            write!(out, ",")?;
+        }
+        write!(out, "{}", param.0)?;
+    }
+    write!(out, "]")?;
+
+    // exit_cont
+    match func.exit_cont {
+        Some(cont_id) => write!(out, ",\"exit_cont\":{}", cont_id.0)?,
+        None => write!(out, ",\"exit_cont\":null")?,
+    }
+
+    // body
+    write!(out, ",\"body\":[")?;
+    for (i, inst) in func.body.iter().enumerate() {
+        if i > 0 {
+            write!(out, ",")?;
+        }
+        write_inst(inst, out)?;
+    }
+    write!(out, "]")?;
+
+    write!(out, "}}")?;
+    Ok(())
+}
+
+fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
+    match inst {
+        JoinInst::Call {
+            func,
+            args,
+            k_next,
+            dst,
+        } => {
+            write!(out, "{{\"type\":\"call\"")?;
+            write!(out, ",\"func\":{}", func.0)?;
+            write!(out, ",\"args\":[")?;
+            for (i, arg) in args.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{}", arg.0)?;
+            }
+            write!(out, "]")?;
+            match k_next {
+                Some(k) => write!(out, ",\"k_next\":{}", k.0)?,
+                None => write!(out, ",\"k_next\":null")?,
+            }
+            match dst {
+                Some(d) => write!(out, ",\"dst\":{}", d.0)?,
+                None => write!(out, ",\"dst\":null")?,
+            }
+            write!(out, "}}")?;
+        }
+        JoinInst::Jump { cont, args, cond } => {
+            write!(out, "{{\"type\":\"jump\"")?;
+            write!(out, ",\"cont\":{}", cont.0)?;
+            write!(out, ",\"args\":[")?;
+            for (i, arg) in args.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{}", arg.0)?;
+            }
+            write!(out, "]")?;
+            match cond {
+                Some(c) => write!(out, ",\"cond\":{}", c.0)?,
+                None => write!(out, ",\"cond\":null")?,
+            }
+            write!(out, "}}")?;
+        }
+        JoinInst::Ret { value } => {
+            write!(out, "{{\"type\":\"ret\"")?;
+            match value {
+                Some(v) => write!(out, ",\"value\":{}", v.0)?,
+                None => write!(out, ",\"value\":null")?,
+            }
+            write!(out, "}}")?;
+        }
+        JoinInst::Compute(mir_like) => {
+            write!(out, "{{\"type\":\"compute\",\"op\":")?;
+            write_mir_like_inst(mir_like, out)?;
+            write!(out, "}}")?;
+        }
+    }
+    Ok(())
+}
+
+fn write_mir_like_inst<W: Write>(inst: &MirLikeInst, out: &mut W) -> std::io::Result<()> {
+    match inst {
+        MirLikeInst::Const { dst, value } => {
+            write!(out, "{{\"kind\":\"const\"")?;
+            write!(out, ",\"dst\":{}", dst.0)?;
+            match value {
+                ConstValue::Integer(n) => {
+                    write!(out, ",\"value_type\":\"integer\"")?;
+                    write!(out, ",\"value\":{}", n)?;
+                }
+                ConstValue::Bool(b) => {
+                    write!(out, ",\"value_type\":\"bool\"")?;
+                    write!(out, ",\"value\":{}", b)?;
+                }
+                ConstValue::String(s) => {
+                    write!(out, ",\"value_type\":\"string\"")?;
+                    write!(out, ",\"value\":\"{}\"", escape_json_string(s))?;
+                }
+                ConstValue::Null => {
+                    write!(out, ",\"value_type\":\"null\"")?;
+                    write!(out, ",\"value\":null")?;
+                }
+            }
+            write!(out, "}}")?;
+        }
+        MirLikeInst::BinOp { dst, op, lhs, rhs } => {
+            write!(out, "{{\"kind\":\"binop\"")?;
+            write!(out, ",\"dst\":{}", dst.0)?;
+            write!(out, ",\"op\":\"{}\"", binop_to_str(*op))?;
+            write!(out, ",\"lhs\":{}", lhs.0)?;
+            write!(out, ",\"rhs\":{}", rhs.0)?;
+            write!(out, "}}")?;
+        }
+        MirLikeInst::Compare { dst, op, lhs, rhs } => {
+            write!(out, "{{\"kind\":\"compare\"")?;
+            write!(out, ",\"dst\":{}", dst.0)?;
+            write!(out, ",\"op\":\"{}\"", compare_to_str(*op))?;
+            write!(out, ",\"lhs\":{}", lhs.0)?;
+            write!(out, ",\"rhs\":{}", rhs.0)?;
+            write!(out, "}}")?;
+        }
+        MirLikeInst::BoxCall {
+            dst,
+            box_name,
+            method,
+            args,
+        } => {
+            write!(out, "{{\"kind\":\"boxcall\"")?;
+            match dst {
+                Some(d) => write!(out, ",\"dst\":{}", d.0)?,
+                None => write!(out, ",\"dst\":null")?,
+            }
+            write!(out, ",\"box\":\"{}\"", escape_json_string(box_name))?;
+            write!(out, ",\"method\":\"{}\"", escape_json_string(method))?;
+            write!(out, ",\"args\":[")?;
+            for (i, arg) in args.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{}", arg.0)?;
+            }
+            write!(out, "]")?;
+            write!(out, "}}")?;
+        }
+    }
+    Ok(())
+}
+
+fn binop_to_str(op: BinOpKind) -> &'static str {
+    match op {
+        BinOpKind::Add => "add",
+        BinOpKind::Sub => "sub",
+        BinOpKind::Mul => "mul",
+        BinOpKind::Div => "div",
+        BinOpKind::Or => "or",
+        BinOpKind::And => "and",
+    }
+}
+
+fn compare_to_str(op: CompareOp) -> &'static str {
+    match op {
+        CompareOp::Lt => "lt",
+        CompareOp::Le => "le",
+        CompareOp::Gt => "gt",
+        CompareOp::Ge => "ge",
+        CompareOp::Eq => "eq",
+        CompareOp::Ne => "ne",
+    }
+}
+
+/// JSON 文字列のエスケープ
+fn escape_json_string(s: &str) -> String {
+    let mut escaped = String::with_capacity(s.len());
+    for c in s.chars() {
+        match c {
+            '"' => escaped.push_str("\\\""),
+            '\\' => escaped.push_str("\\\\"),
+            '\n' => escaped.push_str("\\n"),
+            '\r' => escaped.push_str("\\r"),
+            '\t' => escaped.push_str("\\t"),
+            c if c.is_control() => {
+                escaped.push_str(&format!("\\u{:04x}", c as u32));
+            }
+            c => escaped.push(c),
+        }
+    }
+    escaped
+}
+
+/// JoinModule を JSON 文字列として返す（テスト用ヘルパー）
+pub fn join_module_to_json_string(module: &JoinModule) -> String {
+    let mut output = Vec::new();
+    write_join_module_as_json(module, &mut output).expect("JSON serialization failed");
+    String::from_utf8(output).expect("Invalid UTF-8 in JSON output")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::mir::join_ir::{JoinContId, JoinFuncId};
+    use crate::mir::ValueId;
+
+    #[test]
+    fn test_empty_module() {
+        let module = JoinModule::new();
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"version\":0"));
+        assert!(json.contains("\"entry\":null"));
+        assert!(json.contains("\"functions\":[]"));
+    }
+
+    #[test]
+    fn test_simple_function() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "test".to_string(), vec![ValueId(100)]);
+        func.body.push(JoinInst::Ret {
+            value: Some(ValueId(100)),
+        });
+        module.add_function(func);
+        module.entry = Some(JoinFuncId::new(0));
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"entry\":0"));
+        assert!(json.contains("\"name\":\"test\""));
+        assert!(json.contains("\"params\":[100]"));
+        assert!(json.contains("\"type\":\"ret\""));
+        assert!(json.contains("\"value\":100"));
+    }
+
+    #[test]
+    fn test_const_instruction() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Compute(MirLikeInst::Const {
+            dst: ValueId(1),
+            value: ConstValue::Integer(42),
+        }));
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"kind\":\"const\""));
+        assert!(json.contains("\"value_type\":\"integer\""));
+        assert!(json.contains("\"value\":42"));
+    }
+
+    #[test]
+    fn test_binop_instruction() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Compute(MirLikeInst::BinOp {
+            dst: ValueId(3),
+            op: BinOpKind::Add,
+            lhs: ValueId(1),
+            rhs: ValueId(2),
+        }));
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"kind\":\"binop\""));
+        assert!(json.contains("\"op\":\"add\""));
+        assert!(json.contains("\"lhs\":1"));
+        assert!(json.contains("\"rhs\":2"));
+    }
+
+    #[test]
+    fn test_call_instruction() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Call {
+            func: JoinFuncId::new(1),
+            args: vec![ValueId(100), ValueId(101)],
+            k_next: Some(JoinContId::new(5)),
+            dst: Some(ValueId(200)),
+        });
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"type\":\"call\""));
+        assert!(json.contains("\"func\":1"));
+        assert!(json.contains("\"args\":[100,101]"));
+        assert!(json.contains("\"k_next\":5"));
+        assert!(json.contains("\"dst\":200"));
+    }
+
+    #[test]
+    fn test_jump_instruction() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Jump {
+            cont: JoinContId::new(3),
+            args: vec![ValueId(10)],
+            cond: Some(ValueId(5)),
+        });
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"type\":\"jump\""));
+        assert!(json.contains("\"cont\":3"));
+        assert!(json.contains("\"cond\":5"));
+    }
+
+    #[test]
+    fn test_boxcall_instruction() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Compute(MirLikeInst::BoxCall {
+            dst: Some(ValueId(10)),
+            box_name: "StringBox".to_string(),
+            method: "length".to_string(),
+            args: vec![ValueId(1)],
+        }));
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\"kind\":\"boxcall\""));
+        assert!(json.contains("\"box\":\"StringBox\""));
+        assert!(json.contains("\"method\":\"length\""));
+    }
+
+    #[test]
+    fn test_string_escaping() {
+        let mut module = JoinModule::new();
+        let mut func = JoinFunction::new(JoinFuncId::new(0), "main".to_string(), vec![]);
+        func.body.push(JoinInst::Compute(MirLikeInst::Const {
+            dst: ValueId(1),
+            value: ConstValue::String("hello\nworld\"test".to_string()),
+        }));
+        module.add_function(func);
+
+        let json = join_module_to_json_string(&module);
+        assert!(json.contains("\\n"));
+        assert!(json.contains("\\\""));
+    }
+}
diff --git a/src/mir/join_ir/mod.rs b/src/mir/join_ir/mod.rs
index 155e1654..036d08db 100644
--- a/src/mir/join_ir/mod.rs
+++ b/src/mir/join_ir/mod.rs
@@ -30,6 +30,9 @@ pub mod lowering;
 // Phase 29 L-5.2: Progress verification
 pub mod verify;
 
+// Phase 30.x: JSON serialization (jsonir v0)
+pub mod json;
+
 // Re-export lowering functions for backward compatibility
 pub use lowering::{
     lower_funcscanner_trim_to_joinir, lower_min_loop_to_joinir, lower_skip_ws_to_joinir,
diff --git a/src/tests/joinir_json_min.rs b/src/tests/joinir_json_min.rs
new file mode 100644
index 00000000..0d3d84a3
--- /dev/null
+++ b/src/tests/joinir_json_min.rs
@@ -0,0 +1,232 @@
+//! JoinIR JSON シリアライズテスト (Phase 30.x)
+//!
+//! 手動構築した JoinIR を JSON に変換し、構造の妥当性を検証する。
+
+use crate::mir::join_ir::json::join_module_to_json_string;
+use crate::mir::join_ir::{
+    BinOpKind, CompareOp, ConstValue, JoinContId, JoinFuncId, JoinFunction, JoinInst, JoinModule,
+    MirLikeInst,
+};
+use crate::mir::ValueId;
+
+/// 手動構築した JoinModule の JSON 出力テスト（NYASH_JOINIR_EXPERIMENT 不要）
+#[test]
+fn test_manual_joinir_json() {
+    // skip_ws 相当の JoinIR を手動で構築
+    let mut module = JoinModule::new();
+
+    // skip 関数: skip(s) -> i
+    let mut skip_func = JoinFunction::new(
+        JoinFuncId::new(0),
+        "skip".to_string(),
+        vec![ValueId(3000)], // s
+    );
+
+    // i_init = 0
+    skip_func.body.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: ValueId(3001),
+        value: ConstValue::Integer(0),
+    }));
+
+    // n = s.length()
+    skip_func.body.push(JoinInst::Compute(MirLikeInst::BoxCall {
+        dst: Some(ValueId(3002)),
+        box_name: "StringBox".to_string(),
+        method: "length".to_string(),
+        args: vec![ValueId(3000)],
+    }));
+
+    // loop_step(s, i_init, n) - 末尾呼び出し
+    skip_func.body.push(JoinInst::Call {
+        func: JoinFuncId::new(1),
+        args: vec![ValueId(3000), ValueId(3001), ValueId(3002)],
+        k_next: None,
+        dst: None,
+    });
+
+    module.add_function(skip_func);
+
+    // loop_step 関数: loop_step(s, i, n) -> i
+    let mut loop_step = JoinFunction::new(
+        JoinFuncId::new(1),
+        "loop_step".to_string(),
+        vec![ValueId(3100), ValueId(3101), ValueId(3102)], // s, i, n
+    );
+
+    // cond = i < n
+    loop_step.body.push(JoinInst::Compute(MirLikeInst::Compare {
+        dst: ValueId(3103),
+        op: CompareOp::Lt,
+        lhs: ValueId(3101),
+        rhs: ValueId(3102),
+    }));
+
+    // 条件分岐（簡略化: 条件付き ret）
+    loop_step.body.push(JoinInst::Ret {
+        value: Some(ValueId(3101)),
+    });
+
+    module.add_function(loop_step);
+    module.entry = Some(JoinFuncId::new(0));
+
+    // JSON に変換
+    let json = join_module_to_json_string(&module);
+
+    // 構造チェック
+    assert!(json.contains("\"version\":0"));
+    assert!(json.contains("\"entry\":0"));
+    assert!(json.contains("\"name\":\"skip\""));
+    assert!(json.contains("\"name\":\"loop_step\""));
+
+    // 命令チェック
+    assert!(json.contains("\"kind\":\"const\""));
+    assert!(json.contains("\"kind\":\"boxcall\""));
+    assert!(json.contains("\"kind\":\"compare\""));
+    assert!(json.contains("\"type\":\"call\""));
+    assert!(json.contains("\"type\":\"ret\""));
+
+    // 値チェック
+    assert!(json.contains("\"value_type\":\"integer\""));
+    assert!(json.contains("\"value\":0"));
+    assert!(json.contains("\"box\":\"StringBox\""));
+    assert!(json.contains("\"method\":\"length\""));
+    assert!(json.contains("\"op\":\"lt\""));
+
+    eprintln!("[joinir/json] manual JoinIR JSON output:");
+    eprintln!("{}", json);
+}
+
+/// 全命令タイプの JSON 出力をカバーするテスト
+#[test]
+fn test_all_instruction_types_json() {
+    let mut module = JoinModule::new();
+    let mut func = JoinFunction::new(JoinFuncId::new(0), "all_types".to_string(), vec![]);
+
+    // Const - Integer
+    func.body.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: ValueId(1),
+        value: ConstValue::Integer(42),
+    }));
+
+    // Const - Bool
+    func.body.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: ValueId(2),
+        value: ConstValue::Bool(true),
+    }));
+
+    // Const - String
+    func.body.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: ValueId(3),
+        value: ConstValue::String("hello".to_string()),
+    }));
+
+    // Const - Null
+    func.body.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: ValueId(4),
+        value: ConstValue::Null,
+    }));
+
+    // BinOp - all types
+    for (i, op) in [
+        BinOpKind::Add,
+        BinOpKind::Sub,
+        BinOpKind::Mul,
+        BinOpKind::Div,
+        BinOpKind::Or,
+        BinOpKind::And,
+    ]
+    .iter()
+    .enumerate()
+    {
+        func.body.push(JoinInst::Compute(MirLikeInst::BinOp {
+            dst: ValueId(10 + i as u32),
+            op: *op,
+            lhs: ValueId(1),
+            rhs: ValueId(1),
+        }));
+    }
+
+    // Compare - all types
+    for (i, op) in [
+        CompareOp::Lt,
+        CompareOp::Le,
+        CompareOp::Gt,
+        CompareOp::Ge,
+        CompareOp::Eq,
+        CompareOp::Ne,
+    ]
+    .iter()
+    .enumerate()
+    {
+        func.body.push(JoinInst::Compute(MirLikeInst::Compare {
+            dst: ValueId(20 + i as u32),
+            op: *op,
+            lhs: ValueId(1),
+            rhs: ValueId(1),
+        }));
+    }
+
+    // BoxCall
+    func.body.push(JoinInst::Compute(MirLikeInst::BoxCall {
+        dst: Some(ValueId(30)),
+        box_name: "TestBox".to_string(),
+        method: "test_method".to_string(),
+        args: vec![ValueId(1), ValueId(2)],
+    }));
+
+    // Call
+    func.body.push(JoinInst::Call {
+        func: JoinFuncId::new(1),
+        args: vec![ValueId(1)],
+        k_next: Some(JoinContId::new(2)),
+        dst: Some(ValueId(40)),
+    });
+
+    // Jump (conditional)
+    func.body.push(JoinInst::Jump {
+        cont: JoinContId::new(3),
+        args: vec![ValueId(1)],
+        cond: Some(ValueId(2)),
+    });
+
+    // Jump (unconditional)
+    func.body.push(JoinInst::Jump {
+        cont: JoinContId::new(4),
+        args: vec![],
+        cond: None,
+    });
+
+    // Ret with value
+    func.body.push(JoinInst::Ret {
+        value: Some(ValueId(1)),
+    });
+
+    module.add_function(func);
+
+    // JSON に変換
+    let json = join_module_to_json_string(&module);
+
+    // 全 BinOp タイプをチェック
+    assert!(json.contains("\"op\":\"add\""));
+    assert!(json.contains("\"op\":\"sub\""));
+    assert!(json.contains("\"op\":\"mul\""));
+    assert!(json.contains("\"op\":\"div\""));
+    assert!(json.contains("\"op\":\"or\""));
+    assert!(json.contains("\"op\":\"and\""));
+
+    // 全 Compare タイプをチェック
+    assert!(json.contains("\"op\":\"lt\""));
+    assert!(json.contains("\"op\":\"le\""));
+    assert!(json.contains("\"op\":\"gt\""));
+    assert!(json.contains("\"op\":\"ge\""));
+    assert!(json.contains("\"op\":\"eq\""));
+    assert!(json.contains("\"op\":\"ne\""));
+
+    // 全 ConstValue タイプをチェック
+    assert!(json.contains("\"value_type\":\"integer\""));
+    assert!(json.contains("\"value_type\":\"bool\""));
+    assert!(json.contains("\"value_type\":\"string\""));
+    assert!(json.contains("\"value_type\":\"null\""));
+
+    eprintln!("[joinir/json] all_instruction_types test passed");
+}
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index e977c86b..31cdd8c9 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -6,6 +6,7 @@ pub mod identical_exec;
 pub mod identical_exec_collections;
 pub mod identical_exec_instance;
 pub mod identical_exec_string;
+pub mod joinir_json_min; // Phase 30.x: JoinIR JSON シリアライズテスト
 pub mod joinir_runner_min; // Phase 27.2: JoinIR 実行器 A/B 比較テスト
 pub mod joinir_runner_standalone; // Phase 27-shortterm S-3.2: JoinIR Runner 単体テスト
 pub mod joinir_vm_bridge_skip_ws; // Phase 27-shortterm S-4.4: JoinIR → Rust VM Bridge A/B Test