refactor(mir): Phase 139-P3-A - CapabilityTag enum 定義

- CapabilityTag enum を定義（8 variants）
- to_tag() メソッドでエラータグ文字列に変換
- description() メソッドで人間可読な説明を提供
- legacy capability_tags モジュールを deprecated マーク（P3-B で削除予定）
- 型安全性向上：コンパイル時エラー検出、IDE 支援

src/mir/loop_canonicalizer/capability_guard.rs

@@ -66,17 +66,75 @@ impl RoutingDecision {
 }
 
 // ============================================================================
-// Capability Tags
+// Capability Tags (Type-Safe Enum)
 // ============================================================================
 
-/// Capability tags - Standardized vocabulary for Fail-Fast reasons
+/// Capability tag - Type-safe vocabulary for pattern requirements
 ///
-/// These constants define the capabilities required by different patterns.
+/// Each tag represents a specific capability that a loop pattern requires.
-/// When a loop lacks a required capability, it uses the corresponding tag
+/// Using an enum (instead of string constants) provides:
-/// to explain why it cannot be lowered by that pattern.
+/// - Compile-time error detection for typos
+/// - IDE auto-completion
+/// - Exhaustiveness checking in match expressions
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum CapabilityTag {
+    /// Requires: Carrier update is constant step (i = i + const)
+    ConstStep,
+    /// Requires: Single break point only
+    SingleBreak,
+    /// Requires: Single continue point only
+    SingleContinue,
+    /// Requires: Pure header condition (no side effects)
+    PureHeader,
+    /// Requires: Outer local condition variable
+    OuterLocalCond,
+    /// Requires: Complete exit bindings
+    ExitBindings,
+    /// Requires: Carrier promotion support
+    CarrierPromotion,
+    /// Requires: Consistent break value types
+    BreakValueType,
+}
+
+impl CapabilityTag {
+    /// Error message tag for contract_checks integration
+    pub fn to_tag(&self) -> &'static str {
+        match self {
+            Self::ConstStep => "CAP_MISSING_CONST_STEP",
+            Self::SingleBreak => "CAP_MISSING_SINGLE_BREAK",
+            Self::SingleContinue => "CAP_MISSING_SINGLE_CONTINUE",
+            Self::PureHeader => "CAP_MISSING_PURE_HEADER",
+            Self::OuterLocalCond => "CAP_MISSING_OUTER_LOCAL_COND",
+            Self::ExitBindings => "CAP_MISSING_EXIT_BINDINGS",
+            Self::CarrierPromotion => "CAP_MISSING_CARRIER_PROMOTION",
+            Self::BreakValueType => "CAP_MISSING_BREAK_VALUE_TYPE",
+        }
+    }
+
+    /// Human-readable description
+    pub fn description(&self) -> &'static str {
+        match self {
+            Self::ConstStep => "Carrier update is constant step (i = i + const)",
+            Self::SingleBreak => "break statement appears in single location only",
+            Self::SingleContinue => "continue statement appears in single location only",
+            Self::PureHeader => "Loop header condition has no side effects",
+            Self::OuterLocalCond => "Condition variable is defined in outer scope",
+            Self::ExitBindings => "Exit bindings are complete (no missing values)",
+            Self::CarrierPromotion => "LoopBodyLocal can be promoted to carrier",
+            Self::BreakValueType => "break value types are consistent across all branches",
+        }
+    }
+}
+
+// ============================================================================
+// Legacy Capability Tags (Deprecated - will be removed in Phase 139-P3-B)
+// ============================================================================
+
+/// Legacy capability tags - Standardized vocabulary for Fail-Fast reasons
 ///
-/// NOTE: This module will be deprecated in Phase 139 in favor of the
+/// ⚠️ DEPRECATED: Use CapabilityTag enum instead for type safety.
-/// CapabilityTag enum for type safety.
+/// This module will be removed in Phase 139-P3-B.
+#[deprecated(since = "Phase 139-P3-A", note = "Use CapabilityTag enum instead")]
 pub mod capability_tags {
     /// Requires: Carrier update is constant step (`i = i + const`)
     pub const CAP_MISSING_CONST_STEP: &str = "CAP_MISSING_CONST_STEP";