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/01-JoinIR-Selfhost-INDEX.md

@@ -42,7 +42,14 @@ JoinIR の箱構造と責務、ループ/if の lowering パターンを把握
    - `docs/development/current/main/phase81-pattern2-exitline-contract.md`（promoted carriers の ExitLine 契約検証）
 5. Boxification feedback（Phase 78–85 の振り返りと Phase 86 推奨）
    - `docs/development/current/main/phase78-85-boxification-feedback.md`
-4. 代表的な Phase 文書（現役ラインとの接点だけ絞ったもの）
+6. Phase 86: Carrier Init Builder + Error Tags ✅
+   - **Status**: COMPLETE (2025-12-13)
+   - **Modules**:
+     - `src/mir/builder/control_flow/joinir/merge/carrier_init_builder.rs` (+8 tests)
+     - `src/mir/join_ir/lowering/error_tags.rs` (+5 tests)
+   - **Achievements**: SSOT 確立（CarrierInit → ValueId 生成統一、エラータグ中央化、DebugOutputBox 完全移行）
+   - **Impact**: 987/987 tests PASS, +13 unit tests, Single Responsibility validated
+7. 代表的な Phase 文書（現役ラインとの接点だけ絞ったもの）
    - `docs/development/current/main/phase33-16-INDEX.md`
    - `docs/development/current/main/phase33-17-joinir-modularization-analysis.md`
    - `docs/development/current/main/phase183-selfhost-depth2-joinir-status.md`