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/phase212-5-implementation-complete.md

@@ -232,3 +232,5 @@ Pattern 3 lowerer (`lower_loop_with_if_phi_pattern`) is currently a **test-only
 **Next**: Phase 213 will generalize Pattern 3 lowerer to read actual AST conditions and update expressions dynamically.
 
 **Phase 212.5: COMPLETE ✅**
+Status: Active  
+Scope: If-sum 実装完了メモ（JoinIR v2）