feat(joinir): Phase 90 P0 - ParseStringComposite pattern

## Composite Pattern
- Continue(escape i+=2) + EarlyReturn(close quote)
- parse_string_composite_pattern.rs (50行、continue_return 再利用)
- 89% コード削減（450行→50行）

## Shape Detection
- BinOp Add const 2 検出（escape 特徴）
- LoopStepInspector 活用（Phase 89 リファクタ成果）

## SSOT Integration
- dev_fixtures.rs に登録
- StepCalculator 再利用（Phase 89-2 成果）

## Tests
- +2 tests (vm_bridge + 期待値 n=10→acc=5)
- normalized_dev: 61→63 passed
- lib: 993 passed (回帰なし)

Impact:
- Reuse over Duplication 実践
- Phase 89 リファクタ成果の完全活用
- 箱化原則 5/5 遵守

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/normalized_joinir_min.rs

@@ -19,6 +19,7 @@ use nyash_rust::mir::join_ir::normalized::fixtures::{
     build_pattern3_json_if_sum_min_structured_for_normalized_dev,
     build_pattern4_continue_min_structured_for_normalized_dev,
     build_pattern_continue_return_min_structured_for_normalized_dev,
+    build_parse_string_composite_min_structured_for_normalized_dev,
     build_selfhost_args_parse_p2_structured_for_normalized_dev,
     build_selfhost_if_sum_p3_ext_structured_for_normalized_dev,
     build_selfhost_if_sum_p3_structured_for_normalized_dev,