feat(mir-builder): add using imports support to json_v0_bridge

Phase 21.8 foundation for MatI64/IntArrayCore integration

Changes:
- Add `imports: HashMap<String, String>` to BridgeEnv
- Extend MapVars::resolve() to check imports and create static box references
- Add BridgeEnv::with_imports() to initialize with using imports map
- Add parse_json_v0_to_module_with_imports() to json_v0_bridge
- Add program_json_to_mir_json_with_imports() to mir_builder.rs
- Maintain backward compatibility via empty HashMap defaults

Next: Wire using extraction from pipeline and test with MatI64

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/host_providers/mir_builder.rs

@@ -1,11 +1,17 @@
 use crate::runner;
 use serde_json::Value as JsonValue;
+use std::collections::HashMap;
 use std::fs;
 use std::io::Write;
 
 /// Convert Program(JSON v0) to MIR(JSON v0) and return it as a String.
 /// Fail-Fast: prints stable tags on stderr and returns Err with the same tag text.
 pub fn program_json_to_mir_json(program_json: &str) -> Result<String, String> {
+    program_json_to_mir_json_with_imports(program_json, HashMap::new())
+}
+
+/// Convert Program(JSON v0) to MIR(JSON v0) with using imports support.
+pub fn program_json_to_mir_json_with_imports(program_json: &str, imports: HashMap<String, String>) -> Result<String, String> {
     // Basic header check
     if !program_json.contains("\"version\"") || !program_json.contains("\"kind\"") {
         let tag = "[mirbuilder/input/invalid] missing version/kind keys";
@@ -13,8 +19,8 @@ pub fn program_json_to_mir_json(program_json: &str) -> Result<String, String> {
         return Err(tag.into());
     }
 
-    // Parse Program(JSON v0) into a MIR Module
+    // Parse Program(JSON v0) into a MIR Module with imports
-    let module = match runner::json_v0_bridge::parse_json_v0_to_module(program_json) {
+    let module = match runner::json_v0_bridge::parse_json_v0_to_module_with_imports(program_json, imports) {
         Ok(m) => m,
         Err(e) => {
             let tag = format!("[mirbuilder/parse/error] {}", e);

src/runner/json_v0_bridge/lowering.rs

@@ -98,10 +98,16 @@ pub(super) struct BridgeEnv {
     pub(super) allow_me_dummy: bool,
     pub(super) me_class: String,
     pub(super) try_result_mode: bool,
+    // Phase 21.8: using imports map (alias -> box_type)
+    pub(super) imports: HashMap<String, String>,
 }
 
 impl BridgeEnv {
     pub(super) fn load() -> Self {
+        Self::with_imports(HashMap::new())
+    }
+
+    pub(super) fn with_imports(imports: HashMap<String, String>) -> Self {
         let trm = crate::config::env::try_result_mode();
         // フェーズM.2: no_phi変数削除
         if crate::config::env::cli_verbose() {
@@ -113,6 +119,7 @@ impl BridgeEnv {
             allow_me_dummy: std::env::var("NYASH_BRIDGE_ME_DUMMY").ok().as_deref() == Some("1"),
             me_class: std::env::var("NYASH_BRIDGE_ME_CLASS").unwrap_or_else(|_| "Main".to_string()),
             try_result_mode: trm,
+            imports,
         }
     }
 }
@@ -250,11 +257,11 @@ pub(super) fn lower_stmt_list_with_vars(
     Ok(cur)
 }
 
-pub(super) fn lower_program(prog: ProgramV0) -> Result<MirModule, String> {
+pub(super) fn lower_program(prog: ProgramV0, imports: std::collections::HashMap<String, String>) -> Result<MirModule, String> {
     if prog.body.is_empty() {
         return Err("empty body".into());
     }
-    let env = BridgeEnv::load();
+    let env = BridgeEnv::with_imports(imports);
     let mut module = MirModule::new("ny_json_v0".into());
     let sig = FunctionSignature {
         name: "main".into(),

src/runner/json_v0_bridge/lowering/expr.rs

@@ -51,6 +51,20 @@ impl<'a> VarScope for MapVars<'a> {
         if let Some(&vid) = self.vars.get(name) {
             return Ok(Some(vid));
         }
+        // Phase 21.8: Check using-imported modules/boxes
+        if let Some(box_type) = env.imports.get(name) {
+            let dst = f.next_value_id();
+            if let Some(bb) = f.get_block_mut(cur_bb) {
+                // Treat as static box reference - create a const string representing the box type
+                bb.add_instruction(MirInstruction::Const {
+                    dst,
+                    value: ConstValue::String(box_type.clone()),
+                });
+            }
+            // Cache the resolution for subsequent uses
+            self.vars.insert(name.to_string(), dst);
+            return Ok(Some(dst));
+        }
         if name == "me" {
             if env.allow_me_dummy {
                 let dst = f.next_value_id();

src/runner/json_v0_bridge/mod.rs

@@ -4,8 +4,13 @@ mod lowering;
 
 use ast::{ProgramV0, StmtV0};
 use lowering::lower_program;
+use std::collections::HashMap;
 
 pub fn parse_json_v0_to_module(json: &str) -> Result<crate::mir::MirModule, String> {
+    parse_json_v0_to_module_with_imports(json, HashMap::new())
+}
+
+pub fn parse_json_v0_to_module_with_imports(json: &str, imports: HashMap<String, String>) -> Result<crate::mir::MirModule, String> {
     let prog: ProgramV0 =
         serde_json::from_str(json).map_err(|e| format!("invalid JSON v0: {}", e))?;
     if crate::config::env::cli_verbose() {
@@ -19,7 +24,7 @@ pub fn parse_json_v0_to_module(json: &str) -> Result<crate::mir::MirModule, Stri
     if prog.version != 0 || prog.kind != "Program" {
         return Err("unsupported IR: expected {version:0, kind:\"Program\"}".into());
     }
-    lower_program(prog)
+    lower_program(prog, imports)
 }
 
 pub fn parse_source_v0_to_module(input: &str) -> Result<crate::mir::MirModule, String> {