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/expr_lowerer.rs

@@ -308,6 +308,58 @@ impl<'env, 'builder, S: ScopeManager> ExprLowerer<'env, 'builder, S> {
     }
 }
 
+// ============================================================================
+// Phase 244: ConditionLoweringBox trait implementation
+// ============================================================================
+
+use super::condition_lowering_box::{ConditionLoweringBox, ConditionContext};
+
+impl<'env, 'builder, S: ScopeManager> ConditionLoweringBox<S> for ExprLowerer<'env, 'builder, S> {
+    /// Phase 244: Implement ConditionLoweringBox trait for ExprLowerer
+    ///
+    /// This allows ExprLowerer to be used interchangeably with other condition
+    /// lowering implementations through the unified ConditionLoweringBox interface.
+    ///
+    /// # Design
+    ///
+    /// This implementation is a thin wrapper around the existing `lower()` method.
+    /// The `ConditionContext` parameter is currently unused because ExprLowerer
+    /// already has access to ScopeManager through its constructor.
+    ///
+    /// # Example
+    ///
+    /// ```ignore
+    /// // Pattern 2: Use ExprLowerer via ConditionLoweringBox trait
+    /// let mut lowerer = ExprLowerer::new(&scope, ExprContext::Condition, &mut builder);
+    ///
+    /// let context = ConditionContext {
+    ///     loop_var_name: "i".to_string(),
+    ///     loop_var_id: ValueId(1),
+    ///     scope: &scope,
+    ///     alloc_value: &mut alloc_fn,
+    /// };
+    ///
+    /// let cond_value = lowerer.lower_condition(&break_cond_ast, &context)?;
+    /// ```
+    fn lower_condition(
+        &mut self,
+        condition: &ASTNode,
+        _context: &ConditionContext<S>,
+    ) -> Result<ValueId, String> {
+        // Delegate to existing lower() method
+        // ConditionContext is unused because ExprLowerer already has scope access
+        self.lower(condition).map_err(|e| e.to_string())
+    }
+
+    /// Phase 244: Check if ExprLowerer supports a given condition pattern
+    ///
+    /// This delegates to the existing `is_supported_condition()` static method,
+    /// allowing callers to check support before attempting lowering.
+    fn supports(&self, condition: &ASTNode) -> bool {
+        Self::is_supported_condition(condition)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;