refactor(mir): Phase 139-P3-B - RoutingDecision を enum 対応 + レガシー削除

- RoutingDecision の missing_caps を Vec<CapabilityTag> に変更（型安全化）
- error_tags は to_tag() メソッドで自動生成
- 全 callsite を enum variant に修正
- capability_tags モジュール（文字列定数群）を完全削除
- 全テスト PASS（型安全性向上を確認）
- フォーマット適用

src/mir/loop_canonicalizer/mod.rs

@@ -34,10 +34,10 @@
 // Module Declarations
 // ============================================================================
 
-mod skeleton_types;
+mod canonicalizer;
 mod capability_guard;
 mod pattern_recognizer;
-mod canonicalizer;
+mod skeleton_types;
 
 // ============================================================================
 // Public Re-exports
@@ -45,20 +45,11 @@ mod canonicalizer;
 
 // Skeleton Types
 pub use skeleton_types::{
-    CarrierRole,
-    CarrierSlot,
-    CapturedSlot,
-    ExitContract,
-    LoopSkeleton,
-    SkeletonStep,
-    UpdateKind,
+    CapturedSlot, CarrierRole, CarrierSlot, ExitContract, LoopSkeleton, SkeletonStep, UpdateKind,
 };
 
 // Capability Guard
-pub use capability_guard::{
-    capability_tags,
-    RoutingDecision,
-};
+pub use capability_guard::{CapabilityTag, RoutingDecision};
 
 // Canonicalization Entry Point
 pub use canonicalizer::canonicalize_loop_expr;
@@ -97,13 +88,12 @@ mod tests {
         assert!(success.is_success());
         assert!(!success.is_fail_fast());
 
-        let fail = RoutingDecision::fail_fast(
-            vec![capability_tags::CAP_MISSING_CONST_STEP],
-            "Test failure".to_string(),
-        );
+        let fail =
+            RoutingDecision::fail_fast(vec![CapabilityTag::ConstStep], "Test failure".to_string());
         assert!(!fail.is_success());
         assert!(fail.is_fail_fast());
         assert_eq!(fail.missing_caps.len(), 1);
+        assert_eq!(fail.missing_caps[0], CapabilityTag::ConstStep);
     }
 
     #[test]