feat(mir): Phase 84-2 CopyTypePropagator for Copy chain type propagation

- Add CopyTypePropagator box (ChatGPT Pro design) for fixed-point
  Copy instruction type propagation
- Integrate into lifecycle.rs before return type inference
- Case D reduced from 12 to 9 (25% reduction)

Implementation:
- src/mir/phi_core/copy_type_propagator.rs: New box with fixed-point loop
- src/mir/phi_core/mod.rs: Add module export
- src/mir/builder/lifecycle.rs: Call propagator before return inference

Test results:
- Baseline: 494 passed, 33 failed (was 489/34)
- Case D: 9 remaining (from 12)
- Unit tests: 4/4 passed

Remaining 9 Case D breakdown:
- GroupA: Loop Edge Copy (7 cases) - PHI incoming needs Copy trace
- GroupB: Multi-level PHI (2 cases) - Recursive PHI resolution needed

Phase 84-3 will address GroupA with Edge Copy tracing in GenericTypeResolver.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/lifecycle.rs

@@ -43,6 +43,8 @@ use crate::mir::join_ir::lowering::type_hint_policy::TypeHintPolicy;
 use crate::mir::join_ir::lowering::generic_type_resolver::GenericTypeResolver;
 // Phase 83: P3-D 既知メソッド戻り値型推論箱
 use crate::mir::join_ir::lowering::method_return_hint::MethodReturnHintBox;
+// Phase 84-2: Copy命令型伝播箱（ChatGPT Pro設計）
+use crate::mir::phi_core::copy_type_propagator::CopyTypePropagator;
 
 // Phase 82: dev ガード用ヘルパー - Case 分類ロジック統一化
 //
@@ -303,6 +305,12 @@ impl super::MirBuilder {
         let mut module = self.current_module.take().unwrap();
         let mut function = self.current_function.take().unwrap();
         function.metadata.value_types = self.value_types.clone();
+        // Phase 84-2: Copy命令型伝播（return型推論の前に実行）
+        //
+        // Loop exit や If merge の edge copy で発生する型欠如を解消する。
+        // Copy チェーン: v1 → v2 → v3 で v1 の型が既知なら v2, v3 にも伝播。
+        CopyTypePropagator::propagate(&function, &mut self.value_types);
+
         // Phase 82-5: lifecycle.rs バグ修正 - terminator の Return のみをチェック
         //   問題: instructions を先に走査すると、中間値（const void 等）を誤って推論対象にしてしまう
         //   解決: terminator の Return のみをチェックし、実際の戻り値を正しく推論する