docs: Phase 138 - ReturnValueLowererBox SSOT documentation

Phase 138 Documentation:
- Created docs/development/current/main/phases/phase-138/README.md
- Goal: Extract return lowering to shared Box for SSOT
- SSOT: common/return_value_lowerer_box.rs
- Supported: Variable, Integer literal, Add expression (x + 2, 5 + 3)
- Implementation: New files, modified files, call sites
- Tests: 5 unit tests + all regressions PASS
- Architecture Impact: Code reduction, maintainability, testability
- Phase 139 準備: Next step clearly indicated (post_if_post_k.rs unification)

10-Now.md Updates:
- Added Phase 138 entry (ReturnValueLowererBox SSOT)
- Implementation details and test coverage
- Design decision: Phase 138 P0 migrated loop paths only
- Link to Phase 138 README

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

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

@@ -1,5 +1,32 @@
 # Self Current Task — Now (main)
 
+## 2025-12-18：Phase 138 完了 ✅
+
+**Phase 138: ReturnValueLowererBox - Return Lowering SSOT**
+- 目的: Return lowering logic を共有 Box として抽出し SSOT を確立
+- 実装:
+  - Created `common/return_value_lowerer_box.rs` (~300 lines)
+  - Migrated `loop_true_break_once.rs` to use Box (2 call sites)
+  - 5 comprehensive unit tests for all patterns
+  - Code reduction: ~115 lines removed from loop_true_break_once.rs
+- SSOT:
+  - `ReturnValueLowererBox::lower_to_value_id()`
+  - Supported: Variable, Integer literal, Add expression (x + 2, 5 + 3)
+  - Fallback: Out-of-scope patterns return `Ok(None)`
+- Tests:
+  - 5 new unit tests: variable, integer literal, add (var+int), add (int+int), fallback (subtract)
+  - All regressions PASS: Phase 137/97/131/135/136
+- Design Decision:
+  - Phase 138 P0 migrated loop paths only
+  - Phase 139 P0 will unify post_if_post_k.rs for complete SSOT
+- Architecture Impact:
+  - Single location for return lowering improvements
+  - Isolated unit tests for return lowering logic
+  - Easy to add new return patterns in one location
+- 入口: `docs/development/current/main/phases/phase-138/README.md`
+
+---
+
 ## 2025-12-18：Phase 137 完了 ✅
 
 **Phase 137: loop(true) break-once with return add expression**