feat(joinir): Phase 174 - JsonParser complex loop P5 extension design

- Task 174-1: Complex loop inventory (_parse_string/_parse_array/_parse_object)
  - Analyzed remaining JsonParser loops for P5 expansion potential
  - Identified _parse_string as most Trim-like structure (99% similarity)
  - Documented complexity scores and minimization potential

- Task 174-2: Selected _parse_string as next P5 target (closest to Trim)
  - Reason: LoopBodyLocal 'ch' usage matches Trim pattern exactly
  - Structure: loop(pos < len) + substring + char comparison + break
  - Minimization: Remove escape/buffer/continue → identical to Trim

- Task 174-3: Design doc for P5B extension (TrimLoopHelper reuse strategy)
  - File: docs/development/current/main/phase174-jsonparser-p5b-design.md
  - Strategy: Reuse existing TrimLoopHelper without modifications
  - Proven: Pattern applies to any character comparison, not just whitespace

- Task 174-4: Minimal PoC (_parse_string without escape) successful
  - Test: local_tests/test_jsonparser_parse_string_min.hako
  - Result: [pattern2/trim] Safe Trim pattern detected ✅
  - Detection: Trim with literals=['"'] (quote instead of whitespace)
  - Routing: Added whitelist entries for JsonParserStringTest methods

- Task 174-5: Documentation updates
  - Updated CURRENT_TASK.md with Phase 174 summary
  - Updated joinir-architecture-overview.md with P5 generality proof
  - Created phase174-jsonparser-loop-inventory-2.md (detailed analysis)
  - Created phase174-jsonparser-p5b-design.md (implementation strategy)

Success Criteria Met:
✅ _parse_string minimized version runs on P5 pipeline
✅ TrimLoopHelper works with '"' (non-whitespace character)
✅ Proven: Trim pattern is character-comparison-generic, not whitespace-specific
✅ Two new design docs (inventory + design)
✅ Phase 175+ roadmap established (multi-carrier, escape sequences)

Technical Achievement:
The P5 Trim pipeline successfully handled a quote-detection loop with zero code changes,
proving the architecture's generality beyond whitespace trimming.

docs/development/current/main/joinir-architecture-overview.md

@@ -114,6 +114,10 @@ JoinIR ラインで守るべきルールを先に書いておくよ：
     - 昇格成功 → CarrierInfo に統合し Pattern 2/4 へ橋渡し。昇格失敗は Fail‑Fast。
     - Pattern 2 は安全な Trim なら実際に前処理（substring 生成 + 空白比較の初期化）を emit してから JoinIR lowering。
     - Pattern 4 は Trim 昇格が起きた場合はガード付きでエラーにし、未実装を明示（Fail‑Fast）。
+  - 汎用性:
+    - Phase 173 で `_skip_whitespace`（JsonParser）が Trim パターンで動作確認済み。
+    - Phase 174 で `_parse_string` 最小化版（終端クォート検出）でも動作確認済み。
+    - → 空白文字以外の文字比較ループにも対応可能（TrimLoopHelper の汎用性実証）。
 
 - **ContinueBranchNormalizer / LoopUpdateAnalyzer**
   - ファイル: