feat(phi): Phase 26-C-1 - LoopSnapshotManager実装完了

- Snapshot一元管理Box実装（402行）
- Preheader/Exit/Continue snapshot統一管理
- 変数変更検出（is_modified）
- 13個の包括的単体テスト全PASS ✅

Box-First理論: Snapshot管理の責任を明確に分離

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

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

src/mir/phi_core/mod.rs

@@ -22,6 +22,9 @@ pub mod loop_var_classifier;
 pub mod phi_input_collector;
 pub mod body_local_phi_builder;
 
+// Phase 26-C: Loop Snapshot & Header PHI Management
+pub mod loop_snapshot_manager;
+
 // Public surface for callers that want a stable path:
 // Phase 1: No re-exports to avoid touching private builder internals.
 // Callers should continue using existing paths. Future phases may expose