refactor(joinir): Phase 244 - ConditionLoweringBox trait unification

Unify condition lowering logic across Pattern 2/4 with trait-based API.

New infrastructure:
- condition_lowering_box.rs: ConditionLoweringBox trait + ConditionContext (293 lines)
- ExprLowerer implements ConditionLoweringBox trait (+51 lines)

Pattern migrations:
- Pattern 2 (loop_with_break_minimal.rs): Use trait API
- Pattern 4 (loop_with_continue_minimal.rs): Use trait API

Benefits:
- Unified condition lowering interface
- Extensible for future lowering strategies
- Clean API boundary between patterns and lowering logic
- Zero code duplication

Test results: 911/911 PASS (+2 new tests)

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

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

docs/development/current/main/phase238-exprlowerer-scope-boundaries.md

@@ -58,3 +58,5 @@ ScopeManager が解決する名前の例:
 - ScopeManager のガード強化（条件式から UpdateEnv へのアクセス禁止を型/モジュール境界で表現）。
 - ConditionEnv 構築を ScopeManager 中心に巻き直す（ConditionEnvBuilder v2 を ScopeManager-front に寄せる）。
 - LAYER_GUARD 的な静的チェックの導入検討。***
+Status: Active  
+Scope: ExprLowerer スコープ境界（JoinIR/ExprLowerer ライン）