feat(joinir): Phase 41-4.1 Add NestedIfMerge JoinInst variant

Add structural support for nested if patterns with PHI-sensitive variables:

1. JoinInst::NestedIfMerge in mod.rs
   - conds: Vec<VarId> (outer to inner conditions)
   - merges: Vec<MergePair> (variable updates at deepest level)
   - k_next: Option<JoinContId> (continuation after merge)

2. JSON serialization in json.rs
   - Type: "nested_if_merge"
   - Fields: conds[], merges[], k_next

3. Runner/Bridge stubs
   - JoinIR Runner: Returns error (use VM Bridge instead)
   - VM Bridge: panic placeholder (41-4.3 will implement)

Target: ParserControlBox.parse_loop() (4-level nested if)

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

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

src/mir/join_ir/json.rs

@@ -180,6 +180,38 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
             write!(out, "]")?;
             write!(out, "}}")?;
         }
+        // Phase 41-4: NestedIfMerge instruction JSON serialization
+        JoinInst::NestedIfMerge { conds, merges, k_next } => {
+            write!(out, "{{\"type\":\"nested_if_merge\"")?;
+            // conds array
+            write!(out, ",\"conds\":[")?;
+            for (i, cond) in conds.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{}", cond.0)?;
+            }
+            write!(out, "]")?;
+            // merges array
+            write!(out, ",\"merges\":[")?;
+            for (i, merge) in merges.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{{")?;
+                write!(out, "\"dst\":{}", merge.dst.0)?;
+                write!(out, ",\"then_val\":{}", merge.then_val.0)?;
+                write!(out, ",\"else_val\":{}", merge.else_val.0)?;
+                write!(out, "}}")?;
+            }
+            write!(out, "]")?;
+            // k_next
+            match k_next {
+                Some(k) => write!(out, ",\"k_next\":{}", k.0)?,
+                None => write!(out, ",\"k_next\":null")?,
+            }
+            write!(out, "}}")?;
+        }
         JoinInst::Compute(mir_like) => {
             write!(out, "{{\"type\":\"compute\",\"op\":")?;
             write_mir_like_inst(mir_like, out)?;

src/mir/join_ir/mod.rs

@@ -322,6 +322,38 @@ pub enum JoinInst {
         args: Vec<VarId>,
     },
 
+    /// Phase 41-4: 深いネスト if の複数変数 merge（else なし）
+    ///
+    /// # Pattern
+    /// ```text
+    /// if cond1 {
+    ///   if cond2 {
+    ///     if cond3 {
+    ///       x = new_val  // modifications only at deepest level
+    ///     }
+    ///   }
+    /// }
+    /// // merge: x = phi(new_val if all conds true, original otherwise)
+    /// ```
+    ///
+    /// # Semantics
+    /// - `conds`: 外側から内側への条件リスト
+    /// - `merges`: 最深レベルでの変数更新
+    ///   - then_val: 全ての conds が true の場合の値
+    ///   - else_val: いずれかの cond が false の場合の値（元の値）
+    /// - MIR 変換時に多段 Branch + PHI を生成
+    ///
+    /// # Target
+    /// ParserControlBox.parse_loop() の 4 レベルネスト if パターン
+    NestedIfMerge {
+        /// 条件リスト（外側から内側へ）
+        conds: Vec<VarId>,
+        /// 変数更新（全条件 true 時 → then_val、いずれか false 時 → else_val）
+        merges: Vec<MergePair>,
+        /// merge 後の継続
+        k_next: Option<JoinContId>,
+    },
+
     /// それ以外の演算は、現行 MIR の算術/比較/boxcall を再利用
     Compute(MirLikeInst),
 }

src/mir/join_ir_runner.rs

@@ -190,6 +190,14 @@ fn execute_function(
                         "MethodCall is not supported in JoinIR Runner (use JoinIR→MIR→VM bridge instead)"
                     ));
                 }
+                // Phase 41-4: NestedIfMerge instruction execution
+                JoinInst::NestedIfMerge { .. } => {
+                    // Phase 41-4: NestedIfMerge は JoinIR Runner では未対応
+                    // JoinIR → MIR 変換経由で VM が実行する
+                    return Err(JoinRuntimeError::new(
+                        "NestedIfMerge is not supported in JoinIR Runner (use JoinIR→MIR→VM bridge instead)"
+                    ));
+                }
             }
         }
 

src/mir/join_ir_vm_bridge.rs

@@ -604,6 +604,17 @@ Call {{\n\
 
                 current_instructions = Vec::new();
             }
+            // Phase 41-4: NestedIfMerge instruction
+            JoinInst::NestedIfMerge { conds, merges, k_next } => {
+                // Phase 41-4.3 で実装予定
+                // TODO: 多段 Branch + PHI を生成
+                // 今は dev フラグ (HAKO_JOINIR_NESTED_IF) でしか到達しないので panic で OK
+                let _ = (conds, merges, k_next); // suppress unused warnings
+                panic!(
+                    "[Phase 41-4] NestedIfMerge is not yet implemented in VM Bridge. \
+                     This instruction is Phase 41-4.3 work-in-progress."
+                );
+            }
         }
     }