docs(joinir): Phase 86 到達点 - Carrier init builder + Error tags SSOT

Phase 74-86 成果を SSOT docs に反映:
- 10-Now.md: Phase 86 achievements summary
- INDEX: carrier_init_builder, error_tags modules documented
- architecture-overview: SSOT module references added

Impact: 987/987 tests, +13 unit tests, 2 new SSOT modules
Modules: carrier_init_builder.rs, error_tags.rs

🤖 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

@@ -17,8 +17,24 @@
 - 残課題（JoinIR ライン）：
   - JoinIR→MIR merge の一般化（複雑な Select/PHI パターンの統合）
   - JsonParserBox など実アプリ側での長期運用テスト
-  - Boxification（箱化）の振り返りと、次の小粒リファクタ（Phase 86 推奨）:
-    - `docs/development/current/main/phase78-85-boxification-feedback.md`
+
+### Phase 86: Carrier Init Builder + Error Tags (2025-12-13) ✅
+
+**Status**: COMPLETE - SSOT 確立完了
+
+**Achievements**:
+- Carrier Initialization Builder: CarrierInit → ValueId 生成を単一関数化 (carrier_init_builder.rs, +8 tests)
+- Error Message Centralization: JoinIR エラータグを SSOT 化 (error_tags.rs, +5 tests)
+- DebugOutputBox Migration: 全 JOINIR_DEBUG 使用箇所を DebugOutputBox に統一
+
+**Impact**:
+- Tests: 987/987 PASS (+13 unit tests from Phase 86)
+- SSOT modules: carrier_init_builder, error_tags (一貫性向上)
+- Code quality: Single Responsibility, Testability First principles validated
+
+**Next**: Phase 87 (LLVM exe line SSOT)
+
+**Reference**: phase78-85-boxification-feedback.md, carrier_init_builder.rs, error_tags.rs
 
 ### Scope / BindingId（dev-only の段階移行ライン）