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/phase200-C-digits-e2e.md

@@ -331,3 +331,5 @@ NYASH_TRACE_PHI=1 NYASH_TRACE_VARMAP=1 ./target/release/hakorune \
 2. **段階適用**: Pattern 2 のみに統合、他パターンは影響なし
 3. **Fail-Fast 維持**: 安全でないパターンは無視（エラーにしない）
 4. **最小変更**: 既存の routing/lowering フローを大幅に変えない
+Status: Active  
+Scope: digits ケースの end-to-end 収束メモ（ConditionEnv ライン）