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>

src/mir/join_ir/lowering/mod.rs

@@ -26,6 +26,7 @@ pub(crate) mod common; // Internal lowering utilities
 pub mod complex_addend_normalizer; // Phase 192: Complex addend normalization (AST preprocessing)
 pub mod digitpos_condition_normalizer; // Phase 224-E: DigitPos condition normalizer (digit_pos < 0 → !is_digit_pos)
 pub mod condition_env; // Phase 171-fix: Condition expression environment
+pub mod condition_lowering_box; // Phase 244: Unified condition lowering interface (trait-based)
 pub mod condition_pattern; // Phase 219-fix: If condition pattern detection (simple vs complex)
 pub mod loop_body_local_env; // Phase 184: Body-local variable environment
 pub mod loop_body_local_init; // Phase 186: Body-local init expression lowering