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/loop_pattern_space.md

@@ -16,7 +16,7 @@
 | C. スキップ       | ① なし ② continue ③ 条件付き cont | 次のイテレーションに「飛ぶ」  |
 | D. PHI 分岐       | ① なし ② if‑PHI ③ match‑PHI      | 条件に応じて値が変わるパターン |
 | E. 条件変数のスコープ | ① OuterLocal ② LoopBodyLocal     | 条件で参照される変数の定義位置 |
-| F. キャリア更新   | ① 単一 ② 複数 ③ 条件付き           | ループ内での状態更新パターン   |
+| F. キャリア更新   | ① 単一 ② 複数 ③ 条件付き           | ループ内での状態更新パターン（✅ Phase 176 で複数対応完了）   |
 
 この 6 軸は、それぞれ 2〜3 通りしかないので、理論上は最大 3×3×3×3×2×3=486 通りの組み合わせになるが、  
 実際に意味のあるパターンはずっと少ない（10〜20 程度）ことが分かった。