fix(builder): 修正案A実装 - emit_unified_call↔emit_box_or_plugin_call再入防止

🎯 無限再帰の構造的防止（修正案A採用）

## 問題
Phase 2リファクタリング後、stack overflow発生:
```
emit_unified_call (emit.rs:15)
  ↓
emit_box_or_plugin_call (utils.rs:136)
  ↓ line 190
emit_unified_call (emit.rs:15) ← 無限ループ！
```

## 修正案A: 再入防止ガード（採用理由）
- B（Math機能削除）: 対処療法で仕様削減 ❌
- C（"birth"特別扱い）: 局所的修正で他Boxに波及 ❌
- A（構造的再入防止）: 根治的アプローチ ✅

## 実装内容

### 1. MirBuilder にフラグ追加
```rust
pub(super) in_unified_boxcall_fallback: bool
```
役割: RouterPolicyでRoute::BoxCallと決めたフォールバック中マーク

### 2. emit_unified_call 側修正 (emit.rs)
```rust
// Route::BoxCall のときだけ
self.in_unified_boxcall_fallback = true;
emit_box_or_plugin_call(...);
self.in_unified_boxcall_fallback = false;
```

### 3. emit_box_or_plugin_call 側修正 (utils.rs)
```rust
if use_unified_env
    && matches!(route, Route::Unified)
    && !self.in_unified_boxcall_fallback  // ← 追加
{
    // emit_unified_call(...) への再入を防止
}
```

## 構造的改善
- RouterPolicyBox の決定を優先
- emit_unified_call → emit_box_or_plugin_call の一方向化
- 「上位の決定を尊重する」という明確なルール

## 残存課題
⚠️ まだstack overflowが残存（別の再帰ルート存在の可能性）
→ 次のステップでstack trace解析が必要

## テスト状況
- Test 1 (Direct VM): ✅ 成功
- Test 2 (Stage-B): ❌ stack overflow（別ルート調査中）
- Test 3 (MIR verification): ✅ 成功

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

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

src/mir/builder.rs

@@ -180,6 +180,14 @@ pub struct MirBuilder {
     pub(super) local_ssa_map: HashMap<(BasicBlockId, ValueId, u8), ValueId>,
     /// BlockSchedule cache: deduplicate materialize copies per (bb, src)
     pub(super) schedule_mat_map: HashMap<(BasicBlockId, ValueId), ValueId>,
+
+    /// Guard flag to prevent re-entering emit_unified_call from BoxCall fallback.
+    /// Used when RouterPolicyBox in emit_unified_call has already decided to
+    /// route a given Method call to BoxCall; emit_box_or_plugin_call must not
+    /// bounce back into the unified path for the same call, otherwise an
+    /// infinite recursion (emit_unified_call → emit_box_or_plugin_call →
+    /// emit_unified_call …) can occur when routing decisions disagree.
+    pub(super) in_unified_boxcall_fallback: bool,
 }
 
 impl MirBuilder {
@@ -232,6 +240,8 @@ impl MirBuilder {
 
             local_ssa_map: HashMap::new(),
             schedule_mat_map: HashMap::new(),
+
+            in_unified_boxcall_fallback: false,
         }
     }
 

src/mir/builder/calls/emit.rs

@@ -108,7 +108,16 @@ impl MirBuilder {
                     eprintln!("[router-guard] {}.{} → BoxCall fallback (recv=%{})", box_name, method, r.0);
                 }
                 let effects = EffectMask::READ.add(Effect::ReadHeap);
-                return self.emit_box_or_plugin_call(dst, *r, method.clone(), None, args, effects);
+                // Prevent BoxCall helper from bouncing back into emit_unified_call
+                // for the same call. RouterPolicyBox has already decided on
+                // Route::BoxCall for this callee, so emit_box_or_plugin_call
+                // must not re-enter the unified path even if its own heuristics
+                // would otherwise choose Unified.
+                let prev_flag = self.in_unified_boxcall_fallback;
+                self.in_unified_boxcall_fallback = true;
+                let res = self.emit_box_or_plugin_call(dst, *r, method.clone(), None, args, effects);
+                self.in_unified_boxcall_fallback = prev_flag;
+                return res;
             }
         }
 

src/mir/builder/utils.rs

@@ -181,7 +181,14 @@ impl super::MirBuilder {
                 );
             }
         }
-        if use_unified_env && matches!(route, crate::mir::builder::router::policy::Route::Unified) {
+        // Unified path from BoxCall helper is only allowed when we are not
+        // already in a BoxCall fallback originating from emit_unified_call.
+        // in_unified_boxcall_fallback is set by emit_unified_call's RouterPolicy
+        // guard when it has already decided that this call must be a BoxCall.
+        if use_unified_env
+            && matches!(route, crate::mir::builder::router::policy::Route::Unified)
+            && !self.in_unified_boxcall_fallback
+        {
             let target = super::builder_calls::CallTarget::Method {
                 box_type,
                 method: method.clone(),