docs: Phase 123 DONE

Updates documentation to reflect Phase 123 completion.

**Updates**:
1. `docs/development/current/main/phases/phase-123/README.md`:
   - Marked all success criteria as complete
   - Added comprehensive DONE section with implementation details
   - Documented key design decisions (graceful degradation, fail-fast, box-first)
   - Added verification commands and next steps (Phase 124)

2. `docs/development/current/main/10-Now.md`:
   - Moved Phase 123 from "Next" to "2025-12-18: Phase 123 完了"
   - Added Phase 123 summary (Return literal, If minimal compare, graceful degradation)
   - Updated "Next" to Phase 124 (Normalized reads facts + Return(Variable))

3. `docs/development/current/main/01-JoinIR-Selfhost-INDEX.md`:
   - Added Phase 123 entry: "Normalized semantics (Return literal + If minimal compare, dev-only)"

**Phase 123 Summary**:
- P0: Docs complete, scope frozen ✅
- P1: Return(Integer literal) working ✅
- P2: Return(Variable) returns Ok(None) (graceful degradation) ✅
- P3: If(minimal compare) generates correct JoinIR ✅
- P4: Integration smoke passing ✅
- P5: Docs updated ✅

**Verification**:
- Unit tests: 8 passed
- Integration smoke: PASS
- Legacy tests: Still passing (graceful degradation confirmed)

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

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

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

@@ -1,12 +1,25 @@
 # Self Current Task — Now (main)
 
-## Next: Phase 123（予定 / 実装中）
+## Next: Phase 124（予定）
+
+**Phase 124: Normalized reads facts + Return(Variable)（dev-only）**
+- Phase 123 で Return(Integer literal) + If(minimal compare) 完了。次は reads facts による変数サポート
+- 対応ノード: Return(Variable), If(complex conditions), 複合式
+- 既定挙動は不変（`joinir_dev_enabled()` のときだけ生成・検証、`joinir_strict_enabled()` で Fail-Fast）
+- 入口（計画）: `docs/development/current/main/phases/phase-124/README.md`
+
+## 2025-12-18：Phase 123 完了 ✅
 
 **Phase 123: if-only Normalized semantics（dev-only）**
-- Phase 122 は "JoinModule生成＋構造検証" まで完了。次は if-only を Normalized JoinModule として意味のある内容にする
-- 対応ノード: Return(Integer literal), Return(Variable)禁止, If(cond_ast)最小Compare
-- 既定挙動は不変（`joinir_dev_enabled()` のときだけ生成・検証、`joinir_strict_enabled()` で Fail-Fast）
-- 入口（計画）: `docs/development/current/main/phases/phase-123/README.md`
+- Return(Integer literal) → `Const + Ret(Some(vid))` 生成
+- Return(Variable) → `Ok(None)` (graceful degradation, Phase 124 で対応)
+- If(minimal compare) → Variable vs Integer literal のみ対応
+- Graceful degradation: Phase 123 制限は `Ok(None)` で legacy に fallback
+- Structured error codes: `[phase123/...]` prefix で明示的エラー
+- Box-first modularization: parse/verify/lower 責務分離
+- Unit tests: 8 tests PASS (including graceful degradation test)
+- Integration smoke: PASS (`phase123_if_only_normalized_semantics_vm.sh`)
+- 入口: `docs/development/current/main/phases/phase-123/README.md`
 
 ## 2025-12-18：Phase 121 完了 ✅