docs(phase256): document P1.13.5 boundary SSOT unification

Phase 256.8.5 Cleanup: Document the completion of boundary SSOT
unification across Pattern4/6/7.

Updates:
- Phase 256 README: Add P1.13.5 section with implementation details
- 10-Now.md: Add P1.13.5 to recent fixes list

Documentation highlights:
- Background: Pattern2 fix needed horizontal expansion
- Implementation: SSOT unified across all patterns (2,3,4,6,7)
- Benefits: Structural prevention of entry param mismatch
- Results: ~40 lines of duplication eliminated, magic numbers removed

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

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

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

@@ -62,6 +62,7 @@
   - P1.5-DBG: boundary entry params の契約チェックを追加（VM実行前 fail-fast）。
   - P1.6: 契約チェックの薄い集約 `run_all_pipeline_checks()` を導入（pipeline の責務を縮退）。
   - P1.13: Pattern2 boundary entry params を `join_module.entry.params` SSOT へ寄せた（ValueId 推測生成の撤去）。
+  - P1.13.5: Boundary SSOT 統一（Pattern4/6/7 横展開 + 共通ヘルパ `get_entry_function()` 抽出）。
 
 ## 2025-12-20：Phase 257（last_index_of early return loop）🔜