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/phase222-if-cond-normalization.md

@@ -235,3 +235,5 @@ pub fn is_if_sum_pattern(&self) -> bool {
 - Phase 219-fix: ConditionPatternBox 初版実装
 - Phase 220-D: loop 条件変数サポート（ConditionEnv 統合）
 - Phase 221: 制約整理（if condition pattern 制約を発見）
+Status: Active  
+Scope: if 条件正規化（JoinIR v2）