docs: Phase 113 if-only partial assign parity

- Create phases/phase-113/README.md (background, fixed pattern, verification)
- Update 10-Now.md with Phase 113 completion
- Update 01-JoinIR-Selfhost-INDEX.md with Phase 113 entry

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

docs/development/current/main/10-Now.md

@@ -1,5 +1,12 @@
 # Self Current Task — Now (main)
 
+## 2025-12-18：Phase 113 完了 ✅
+
+**Phase 113: if-only partial assign parity**
+- if-only（else なし）の片側代入で「保持 merge」パターンを VM/LLVM で固定
+- Fixture: phase113_if_only_partial_assign_min.hako
+- Smoke: VM + LLVM EXE parity 検証済み
+
 ## 2025-12-18：Phase 112 完了 ✅
 
 **Phase 112: StepTree Capability Guard (strict-only)**