feat(joinir): FuncScannerBox.trim/1 JoinIR VM Bridge A/B test

Phase 30.x: JoinIR VM Bridge でPHI問題を設計で解決できることを実証

変更内容:
- src/runner/modes/vm.rs: FuncScannerBox.trim/1 検出時にJoinIR経由実行
- src/mir/join_ir_vm_bridge.rs: Non-tail call サポート追加
  - dst=Some(id) の場合、結果を格納して次の命令へ継続
- src/tests/joinir_vm_bridge_trim.rs: A/Bテスト新規作成
  - メインテスト: Route A (VM) → "void" (PHI bug), Route C (JoinIR) → "abc" ✅
  - エッジケース: 5パターン全てPASS
- src/config/env.rs: joinir_vm_bridge_debug() 追加
- docs: Phase 30 TASKS.md に L-0.4 追加

テスト結果:
  Route A (MIR→VM直接):     "void"  ← PHI バグ
  Route C (MIR→JoinIR→VM):  "abc"   ← 正解 ✅

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

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

src/config/env.rs

@@ -189,6 +189,26 @@ pub fn fail_fast() -> bool {
     env_bool_default("NYASH_FAIL_FAST", true)
 }
 
+// ---- Phase 29/30 JoinIR toggles ----
+/// JoinIR experiment mode. Required for JoinIR-related experimental paths.
+/// Set NYASH_JOINIR_EXPERIMENT=1 to enable.
+pub fn joinir_experiment_enabled() -> bool {
+    env_bool("NYASH_JOINIR_EXPERIMENT")
+}
+
+/// JoinIR VM bridge mode. When enabled with NYASH_JOINIR_EXPERIMENT=1,
+/// specific functions can be executed via JoinIR → VM bridge instead of direct MIR → VM.
+/// Set NYASH_JOINIR_VM_BRIDGE=1 to enable.
+pub fn joinir_vm_bridge_enabled() -> bool {
+    env_bool("NYASH_JOINIR_VM_BRIDGE")
+}
+
+/// JoinIR VM bridge debug output. Enables verbose logging of JoinIR→MIR conversion.
+/// Set NYASH_JOINIR_VM_BRIDGE_DEBUG=1 to enable.
+pub fn joinir_vm_bridge_debug() -> bool {
+    env_bool("NYASH_JOINIR_VM_BRIDGE_DEBUG")
+}
+
 // VM legacy by-name call fallback was removed (Phase 2 complete).
 
 // ---- Phase 11.8 MIR cleanup toggles ----