feat(joinir): Phase 257 P1.1/P1.2/P1.3 - Pattern6 SSOT + LoopHeaderPhi CFG fix

P1.1: Pattern6 false positive fix (SSOT approach)
- can_lower() now calls extract_scan_with_init_parts() for SSOT
- index_of_string/2 no longer triggers false positive
- Graceful fall-through with Ok(None)

P1.2: LoopHeaderPhi CFG-based correction
- Step 0: Manual successor update from terminators
- CFG-based entry predecessor computation (header_preds - latch)
- Multi-entry-pred support (bb0 host + bb10 JoinIR main)
- Explicit host_entry_block addition (emit_jump runs after finalize)

P1.3: Smoke script validation
- phase254_p0_index_of_vm.sh: --verify + VM error detection
- phase257 smokes updated

Acceptance criteria (all PASS):
✅ phase254_p0_index_of_min.hako verify
✅ phase257_p0_last_index_of_min.hako verify
✅ ./tools/smokes/v2/run.sh --profile quick (no Pattern6 false positive)

Technical discovery:
- Host entry block (bb0) Jump set in Phase 6 (after finalize)
- instruction_rewriter bypasses set_terminator(), skips successor update

🤖 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

@@ -6,6 +6,28 @@
 - Phase 143-loopvocab P3+: 条件スコープ拡張（impure conditions 対応）
 - 詳細: `docs/development/current/main/30-Backlog.md`
 
+## 2025-12-20：Phase 257 P1.1/P1.2/P1.3 完了 ✅
+
+- **P1.1**: Pattern6 SSOT fix（false positive 根治）
+  - `can_lower()` が `extract_scan_with_init_parts()` を呼び出して SSOT を確立
+  - `index_of_string/2` の誤検出が解消（`Ok(None)` で graceful fall-through）
+- **P1.2**: LoopHeaderPhi CFG correction（entry predecessor 自動計算）
+  - Step 0: `update_successors_from_terminator()` を手動実行（instruction_rewriter が set_terminator() をバイパスするため）
+  - CFG から entry predecessors を計算（header_preds - latch_block）
+  - 複数 entry preds 対応（bb0 host + bb10 JoinIR main）
+  - host_entry_block を明示的に追加（emit_jump が finalize() 後に実行されるため）
+- **P1.3**: Smoke script validation（既存実装確認）
+  - `phase254_p0_index_of_vm.sh`：`--verify` + VM error detection で false positive 回避
+
+**検証結果**：
+- ✅ `phase254_p0_index_of_min.hako` verify → PASS
+- ✅ `phase257_p0_last_index_of_min.hako` verify → PASS
+- ✅ `./tools/smokes/v2/run.sh --profile quick` → Pattern6 false positive なし（別の未サポートパターンで止まる）
+
+**技術的発見**：
+- JoinIR merge では host entry block (bb0) の Jump が Phase 6 で設定されるため、Phase 4.5 の finalize() 時点で CFG に現れない
+- 解決策：`builder.current_block` を捕捉し、CFG 計算後に明示的に entry_preds へ追加
+
 ## 2025-12-19：Phase 146/147 完了 ✅
 
 - Phase 146 README: `docs/development/current/main/phases/phase-146/README.md`
@@ -70,6 +92,12 @@
 - Phase 257 README: `docs/development/current/main/phases/phase-257/README.md`
 - Goal: `StringUtils.last_index_of/2` を JoinIR で受理し、`--profile quick` を緑に戻す
   - Investigation（最小再現/論点）: `docs/development/current/main/investigations/phase-257-last-index-of-loop-shape.md`
+  - Status: Pattern6 reverse scan + PHI/CFG 安定化は完了（最初の FAIL は次へ移動）
+  - Current first FAIL: `json_lint_vm / StringUtils.is_integer/1`（nested-if + loop pattern, unsupported）
+
+## 2025-12-20：Phase 258（is_integer nested-if + loop）🔜
+
+- Phase 258 README: `docs/development/current/main/phases/phase-258/README.md`
 
 ## 2025-12-19：Phase 254（index_of loop pattern）✅ 完了（Blocked by Phase 255）