feat(phi): Phase 26-C-2 - HeaderPhiBuilder実装完了

- Header PHI生成専門Box実装（563行）
- Pinned/Carrier変数PHI管理
- PhiInputCollector統合でseal処理サポート
- 13個の包括的単体テスト全PASS ✅

Box-First理論: Header PHI生成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

@@ -24,6 +24,7 @@ pub mod body_local_phi_builder;
 
 // Phase 26-C: Loop Snapshot & Header PHI Management
 pub mod loop_snapshot_manager;
+pub mod header_phi_builder;
 
 // Public surface for callers that want a stable path:
 // Phase 1: No re-exports to avoid touching private builder internals.