feat(joinir): Phase 176 Pattern2 multi-carrier lowering complete

Task 176-1: Pattern2 limitation investigation
- Identified 10 limitation points where only position carrier was handled
- Added TODO markers for Phase 176-2/3 implementation
- Created phase176-pattern2-limitations.md documentation

Task 176-2: CarrierUpdateLowerer helper implementation
- Implemented emit_carrier_update() helper function
- Supports CounterLike and AccumulationLike UpdateExpr patterns
- Added 6 unit tests (all passing)
- Fail-Fast error handling for carrier/variable not found

Task 176-3: Pattern2 lowerer multi-carrier extension
- Extended header PHI generation for all carriers
- Implemented loop update for all carriers using emit_carrier_update()
- Extended ExitLine/ExitMeta construction for all carriers
- Updated function call/jump args to include all carriers
- 9/10 tests passing (1 pre-existing test issue)

Task 176-4: E2E testing and bug fixes
- Fixed Trim pattern loop_var_name overwrite bug (pattern2_with_break.rs)
- Fixed InstructionRewriter latch_incoming mapping bug
- All E2E tests passing (RC=0): pos + result dual-carrier loops work
- test_jsonparser_parse_string_min2.hako verified

Task 176-5: Documentation updates
- Created phase176-completion-report.md
- Updated phase175-multicarrier-design.md with completion status
- Updated joinir-architecture-overview.md roadmap
- Updated CURRENT_TASK.md with Phase 176 completion + Phase 177 TODO
- Updated loop_pattern_space.md F-axis (multi-carrier support complete)

Technical achievements:
- Pattern2 now handles single/multiple carriers uniformly
- CarrierInfo architecture proven to work end-to-end
- Two critical bugs fixed (loop_var overwrite, latch_incoming mapping)
- No regressions in existing tests

Next: Phase 177 - Apply to JsonParser _parse_string full implementation

docs/development/current/main/phase175-multicarrier-design.md

@@ -251,9 +251,30 @@ bb12:  // Exit block
 
 ---
 
-## 5. References
+## 5. Phase 176 で解決済み ✅ (2025-12-08)
+
+**実装内容**:
+- Pattern2 lowerer を全キャリア対応に拡張
+- ヘッダ PHI / ループ更新 / ExitLine で複数キャリアを正しく処理
+- CarrierUpdateLowerer ヘルパで UpdateExpr → MIR 変換を統一
+
+**修正されたバグ**:
+1. Trim pattern で loop_var_name が上書きされていた問題（pattern2_with_break.rs）
+2. InstructionRewriter が loop_var を exit_bindings から除外していなかった問題
+
+**テスト結果**:
+- ✅ 2キャリア E2E テスト全てパス（pos + result）
+- ✅ 回帰テストなし
+- ✅ Trim pattern も正常動作
+
+**次のステップ**: Phase 177 で JsonParser の複雑ループへ拡張
+
+---
+
+## 6. References
 
 - **Phase 170**: LoopUpdateSummary design
 - **Phase 171**: LoopBodyCarrierPromoter implementation
 - **Phase 174**: P5 minimal PoC (quote detection only)
+- **Phase 176**: Pattern2 multi-carrier implementation ([phase176-completion-report.md](phase176-completion-report.md))
 - **Pattern Space**: [docs/development/current/main/loop_pattern_space.md](loop_pattern_space.md)