refactor(joinir): Phase 89 リファクタリング - 2) ShapeCapability分類修正

変更内容:
- 新規 capability 追加: P4ContinueEarlyReturn
  - Continue + Early Return 専用の capability
  - P4ContinueSkipWs と明確に分離
- PatternContinueReturnMinimal のマッピング修正
  - P4ContinueSkipWs → P4ContinueEarlyReturn に変更
- is_p2_core_capability に P4ContinueEarlyReturn を追加

理由:
- P4ContinueSkipWs は「Continue のみ」のパターン
- PatternContinueReturnMinimal は「Continue + Early Return」の合成形
- 将来の拡張時に混乱を避けるため、今のうちに分離

テスト結果:
- lib tests: 993 passed (回帰なし)
- normalized_dev tests: 61 passed / 1 failed (ベースライン維持)

Generated with Claude Code

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

src/mir/join_ir/normalized/shape_guard.rs

@@ -25,6 +25,9 @@ pub enum ShapeCapabilityKind {
     /// P4 Continue (skip whitespace) family
     P4ContinueSkipWs,
 
+    /// P4 Continue + Early Return family (Phase 89)
+    P4ContinueEarlyReturn,
+
     /// Selfhost P2 core (token scan)
     SelfhostP2Core,
 
@@ -220,8 +223,8 @@ pub fn capability_for_shape(shape: &NormalizedDevShape) -> ShapeCapability {
         // Phase 54: selfhost P2/P3 shape growth
         SelfhostVerifySchemaP2 => SelfhostP2Core,
         SelfhostDetectFormatP3 => SelfhostP3IfSum,
-        // Phase 89: Continue + Early Return pattern (dev-only, maps to P4 family)
-        PatternContinueReturnMinimal => P4ContinueSkipWs,
+        // Phase 89: Continue + Early Return pattern (dev-only, dedicated capability)
+        PatternContinueReturnMinimal => P4ContinueEarlyReturn,
     };
 
     ShapeCapability::new(kind)
@@ -271,6 +274,7 @@ pub fn is_p2_core_capability(cap: &ShapeCapability) -> bool {
             | P2MidParseNumber
             | P3IfSum
             | P4ContinueSkipWs
+            | P4ContinueEarlyReturn
             | SelfhostP2Core
             | SelfhostP3IfSum
     )