feat(joinir): Phase 172 - Trim pattern JoinIR lowering implementation

Implement actual MIR generation for Trim pattern loops, enabling
LoopBodyLocal variables to work through bool carrier promotion.

## Implementation (pattern2_with_break.rs)
- emit_whitespace_check(): Generate OR chain for whitespace comparison
- extract_substring_args(): Extract s, start from substring call
- Initial carrier generation: ch0 = s.substring(start, start+1)
- Whitespace check: is_ch_match0 = (ch0 == " " || "\t" || "\n" || "\r")
- ConditionEnv integration: Register carrier for JoinIR condition
- Break condition replacement: !is_ch_match instead of original ch checks

## Architecture
- Host MIR: substring calls, OR chain evaluation, BoxCall
- JoinIR: Only sees bool carrier for break control
- No new JoinIR instructions added

## Documentation
- phase172-trim-lowering-impl.md: Design and implementation details
- loop_pattern_space.md: Analysis of all loop pattern combinations

Test: Trim loops compile and generate JoinIR successfully
Build: 0 errors, clean compilation

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

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

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

@@ -247,3 +247,7 @@ JoinIR は Rust 側だけでなく、将来的に .hako selfhost コンパイラ
   - 185–188: Strict mode / LoopBuilder 削除 / Pattern1–4 基盤
   - 189–193: Multi-function merge / Select bridge / ExitLine 箱化
   - 171–172 / 33‑10/13: ConditionEnv, ConditionBinding, JoinFragmentMeta, ExitLineRefactor 等
+- `docs/development/current/main/loop_pattern_space.md`
+  - JoinIR ループパターン空間の整理メモ。  
+    どの軸（継続条件 / break / continue / PHI / 条件変数スコープ / 更新パターン）でパターンを分けるか、  
+    そして P1–P4 / Trim(P5) の位置づけと、今後追加候補のパターン一覧がまとまっている。