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/phase236-exprlowerer-integration.md

@@ -61,3 +61,5 @@
 
 - Phase 238-EX で Scope/Env の境界ルールを文書化し、ExprLowerer/ScopeManager/ConditionEnv/LoopBodyLocalEnv/UpdateEnv の責務を固定する予定。
 - 参照: `phase238-exprlowerer-scope-boundaries.md`（境界ルールと将来のガード案）。***
+Status: Active  
+Scope: ExprLowerer 統合（JoinIR/ExprLowerer ライン）