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/phase213-if-sum-implementation.md

@@ -96,3 +96,5 @@ This is a pre-existing issue in the JoinIR → MIR conversion pipeline (Phase 33
 - Dispatcher: `src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs`
 - Pattern detection: `src/mir/join_ir/lowering/loop_update_summary.rs`
 - Pipeline context: `src/mir/builder/control_flow/joinir/patterns/pattern_pipeline.rs`
+Status: Active  
+Scope: If-sum 実装（JoinIR v2）