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/phase216-selfhost-if-sum-production.md

@@ -176,3 +176,5 @@ Phase 216 successfully validates that Pattern 3 if-sum implementation is product
 - ✅ No additional bugs found
 
 The complete Phase 213-215 work (AST-based if-sum lowering + ExprResult exit contract) is now validated on production test case. Ready to proceed with multi-carrier and nested patterns (Phase 217+).
+Status: Active  
+Scope: selfhost If-sum 本番適用（JoinIR v2）