refactor(joinir): Phase 44 - Capability-based shape guard

Refactor shape detection from function-name-based to capability-based
architecture for better extensibility and maintainability.

Key changes:
- ShapeCapabilityKind enum: P2CoreSimple/SkipWs/Atoi/ParseNumber
- ShapeCapability: Capability descriptor with future extensibility
- Helper functions:
  - capability_for_shape(): Map shape to capability
  - is_canonical_shape(): Exact shape-level check
  - is_p2_core_capability(): Broad capability family check
  - is_supported_by_normalized(): Normalized dev support

Benefits:
- Extensibility: Easy to add new capability kinds
- Clarity: Shape purpose explicit in kind names
- Maintainability: Centralized mapping vs scattered ifs
- Future-ready: Infrastructure for carrier roles, method signatures

Backward compatibility:
- Zero behavioral changes (pure refactoring)
- Canonical set preserved: Pattern2Mini, skip_ws mini/real, atoi mini
- All existing code paths unchanged

Tests: 937/937 PASS
Files: +78 lines (shape_guard.rs), design doc created

CURRENT_TASK.md

@@ -66,6 +66,11 @@
   - `current_joinir_mode()` でモード取得、bridge/runner で `normalized_dev_enabled()` → mode pattern matching に移行。
   - Canonical P2-Core は mode 無視で常に Normalized→MIR(direct)、それ以外は mode に従う統一ルーティング。
   - 937/937 tests PASS（既存挙動完全保持のリファクタ）。
+- **Phase 44-SHAPE-CAP（実装済み✅ 2025-12-12）**:
+  - Shape検出を capability-based に変更: `NormalizedDevShape` → `ShapeCapability` 抽象化層導入。
+  - `ShapeCapabilityKind` 4種: P2CoreSimple / P2CoreSkipWs / P2CoreAtoi / P2MidParseNumber。
+  - Shape-level (`is_canonical_shape`) と Capability-level (`is_p2_core_capability`) の二層 API でパターン拡張性を確保。
+  - 既存挙動完全保持（canonical set: Pattern2Mini, skip_ws mini/real, atoi mini のまま）、937/937 tests PASS。
 
 ### 1. いまコード側で意識しておきたいフォーカス