feat(mir): Phase 137-5 - Decision Policy SSOT化完了

## 目的
Canonicalizer の RoutingDecision.chosen を「lowerer 選択の最終結果」にする
（構造クラス名ではなく ExitContract ベースの決定）

## 実装内容

### 1. Canonicalizer の決定ロジック修正
- `src/mir/loop_canonicalizer/mod.rs`
  - `skip_whitespace` パターン認識で ExitContract (has_break=true) を考慮
  - Pattern3IfPhi → Pattern2Break に修正（構造は似ているが break あり）
  - 単体テスト更新（Pattern2Break 期待に変更）

### 2. Parity 検証テスト修正
- `src/mir/builder/control_flow/joinir/routing.rs`
  - `test_parity_check_mismatch_detected` → `test_parity_check_skip_whitespace_match`
  - Canonicalizer と Router の一致を検証（ミスマッチ検出からマッチ検証へ）
  - Phase 137-5 の SSOT 原則を反映

### 3. ドキュメント更新
- `docs/development/current/main/design/loop-canonicalizer.md`
  - Phase 137-5: Decision Policy SSOT セクション追加
  - ExitContract 優先の原則を明記
  - skip_whitespace の例を追加

- `docs/development/current/main/phases/phase-137/README.md`
  - Phase 4 完了マーク追加
  - Phase 5 完了セクション追加（実装・検証・効果）

## 検証結果
- ✅ 単体テスト: `cargo test --release --lib loop_canonicalizer::tests` (11/11 passed)
- ✅ Parity テスト: `cargo test --release --lib 'routing::tests::test_parity'` (2/2 passed)
- ✅ Strict モード: `HAKO_JOINIR_STRICT=1` で skip_whitespace parity OK

## 効果
- Router と Canonicalizer の pattern 選択が一致
- ExitContract が pattern 決定の SSOT として明確化
- 構造的特徴（if-else 等）は notes に記録（将来拡張に備える）

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/routing.rs

@@ -351,7 +351,7 @@ mod tests {
     }
 
     #[test]
-    fn test_parity_check_mismatch_detected() {
+    fn test_parity_check_skip_whitespace_match() {
         use crate::mir::loop_canonicalizer::canonicalize_loop_expr;
         use crate::mir::builder::control_flow::joinir::patterns::ast_feature_extractor as ast_features;
 
@@ -373,18 +373,20 @@ mod tests {
         let features = ast_features::extract_features(condition, body, has_continue, has_break);
         let actual_pattern = crate::mir::loop_pattern_detection::classify(&features);
 
-        // Verify mismatch
-        // Canonicalizer: Pattern3IfPhi (recognizes if-else structure)
-        // Router: Pattern2Break (sees has_break)
+        // Phase 137-5: Verify MATCH (ExitContract policy fix)
+        // Both canonicalizer and router should agree on Pattern2Break
+        // because has_break=true (ExitContract determines pattern choice)
         assert_eq!(
             canonical_pattern,
-            crate::mir::loop_pattern_detection::LoopPatternKind::Pattern3IfPhi
+            crate::mir::loop_pattern_detection::LoopPatternKind::Pattern2Break,
+            "Canonicalizer should choose Pattern2Break for has_break=true"
         );
         assert_eq!(
             actual_pattern,
-            crate::mir::loop_pattern_detection::LoopPatternKind::Pattern2Break
+            crate::mir::loop_pattern_detection::LoopPatternKind::Pattern2Break,
+            "Router should classify as Pattern2Break for has_break=true"
         );
-        assert_ne!(canonical_pattern, actual_pattern, "Phase 137-4: This test verifies mismatch detection");
+        assert_eq!(canonical_pattern, actual_pattern, "Phase 137-5: Canonicalizer and router should agree (SSOT policy)");
     }
 
     #[test]