feat(joinir): Phase 188.3 - Pattern6 (NestedLoopMinimal) 選択ロジック実装

## Phase 188.3 進捗: Phase 2 完了 (6/13 tasks)

### 実装完了 ✅

**Phase 1: Fixture作成**
- apps/tests/phase1883_nested_minimal.hako 追加
  - Add/Compare のみ（乗算なし）
  - 期待 exit code: 9 (3×3 nested loops)
- 既存 lowering で fallback 動作確認

**Phase 2: 選択ロジック (SSOT)**
- LoopPatternContext に step_tree_max_loop_depth フィールド追加
- choose_pattern_kind() に Pattern6 選択ロジック実装:
  1. Cheap check (has_inner_loop)
  2. StepTree 構築 (max_loop_depth 取得)
  3. AST validation (is_pattern6_lowerable)
- pattern6_nested_minimal.rs モジュール作成 (stub)
- LOOP_PATTERNS に Pattern6 entry 追加
- **検証**: Pattern6 が正しく選択される ✅

### 設計原則 (確認済み)

1. **Fail-Fast**: Pattern6 選択後は Ok(None) で逃げない
2. **outer 変数 write-back 検出 → validation false** (Phase 188.4+)
3. **最小実装**: inner local だけ、Pattern1 モデル二重化
4. **cfg! 依存なし**: production で動作

### 検証結果

```
[choose_pattern_kind] has_inner_loop=true
[choose_pattern_kind] max_loop_depth=2
[choose_pattern_kind] is_pattern6_lowerable=true
✅ Pattern6 SELECTED!
```

Stub からの期待エラー:
```
[ERROR] ❌ [Pattern6] Nested loop lowering not yet implemented
```

### 次: Phase 3 (Lowering 実装 - 推定4時間)

残りタスク:
- Phase 3-1: AST 抽出ヘルパー
- Phase 3-2: Validation ヘルパー
- Phase 3-3: Continuation 生成 (outer_step, inner_step, k_inner_exit)
- Phase 3-4: fixture が exit=9 を返すことを検証

### 変更ファイル

**新規**:
- apps/tests/phase1883_nested_minimal.hako
- src/mir/builder/control_flow/joinir/patterns/pattern6_nested_minimal.rs
- docs/development/current/main/phases/phase-188.{1,2,3}/README.md

**変更**:
- src/mir/builder/control_flow/joinir/routing.rs (Pattern6 選択)
- src/mir/builder/control_flow/joinir/patterns/router.rs (Context 拡張)
- src/mir/builder/control_flow/joinir/patterns/mod.rs (module 宣言)

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

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

docs/development/current/main/phases/phase-188.2/README.md

@@ -0,0 +1,133 @@
+# Phase 188.2: StepTree nesting depth SSOT (Option A) — strict Fail-Fast
+
+**Date**: 2025-12-27
+**Status**: ✅ Option A implemented / ❌ Pattern 6 lowering deferred (Phase 188.3+)
+**Goal**: `StepTreeFeatures.max_loop_depth` を SSOT として、strict mode で depth > 2 を明示エラーにする
+
+---
+
+## ✅ Decision: Option A Adopted (2025-12-27)
+
+**Chosen Approach**: StepTreeFeatures Integration
+
+**What Phase 188.2 Implements**:
+- Use `StepTree.features.max_loop_depth` as SSOT for nesting depth
+- Add explicit error for `max_loop_depth > 2` in strict mode (capability_guard.rs)
+- Update NestedLoop capability hint to reflect current support
+
+**Implementation Location**:
+- `src/mir/builder/control_flow/joinir/control_tree_capability_guard.rs` (lines 32-76)
+
+**Status**: Option A depth checking implemented, Option B deferred
+
+---
+
+## What Phase 188.2 delivered
+
+- ✅ strict mode の depth 制約を SSOT 化（`max_loop_depth > 2` は明示エラー）
+- ✅ NestedLoop capability hint を現実に合わせて更新（1-levelは許可、2+ level は depth_exceeded）
+- ✅ unit tests 更新（depth=2 PASS / depth=3 FAIL）
+- ✅ quick/integration は不変（回帰なし）
+
+### Verification (reference)
+
+- quick: `./tools/smokes/v2/run.sh --profile quick`
+- integration selfhost: `./tools/smokes/v2/run.sh --profile integration --filter "selfhost_"`
+
+---
+
+## Background
+
+Phase 188.1 delivered infrastructure (capability allowlist, enum, stub module), but **detection and lowering are NOT implemented** due to architectural limitations.
+
+**See**: `docs/development/current/main/phases/phase-188.1/README.md` § Implementation Reality
+
+---
+
+## Design Decision Required (Choose A or B)
+
+### Option A: StepTreeFeatures Integration
+
+**Approach**:
+- Pass `StepTreeFeatures` from StepTree to LoopForm creation
+- Store `max_loop_depth` in LoopForm (add field) or pass via context
+- Use AST-level nesting depth for Pattern 6 detection
+
+**Pros**:
+- Nesting info already exists in StepTree
+- No new detection logic required
+- Fast implementation
+
+**Cons**:
+- AST-level info may diverge from MIR structure (after optimizations)
+- Adding fields to LoopForm breaks separation of concerns
+
+**Estimated Effort**: 1-2 days
+
+---
+
+### Option B: LoopRegion Integration
+
+**Approach**:
+- Implement LoopRegion builder (instantiate parent/child structure)
+- Build LoopRegion tree from MIR control flow graph
+- Compute nesting depth from LoopRegion.parent traversal
+- Replace LoopForm with LoopRegion in lowering pipeline
+
+**Pros**:
+- MIR-level truth (accurate after optimizations)
+- Leverages Phase 32 infrastructure
+- Clean separation (LoopRegion is designed for nesting)
+
+**Cons**:
+- Requires implementing LoopRegion builder (non-trivial)
+- Larger architectural change
+- May affect other loop analysis code
+
+**Estimated Effort**: 1-2 weeks
+
+---
+
+## Planning Session Agenda
+
+**Step 1: Docs-First Design Review**
+- Review Option A vs B trade-offs
+- Consider impact on selfhost_minimal requirements
+- Decide on approach based on scope/timeline
+
+**Step 2: Detailed Implementation Plan**
+- Write step-by-step implementation guide
+- Identify critical files
+- Estimate effort per task
+
+**Step 3: Implementation**
+- Execute chosen approach
+- Test with selfhost_minimal
+- Verify 154/154 PASS maintained
+
+---
+
+## Out of Scope (Phase 188.2)
+
+Still deferred to Phase 188.3+:
+- Break/continue in nested loops (Pattern 2/4 combinations)
+- Multiple inner loops (siblings)
+- 2+ level nesting (depth > 2) の lowering（Phase 188.2 は strict Fail-Fast のみ）
+
+**Rationale**: Phase 188.2 focuses on 1-level nesting detection only (minimal scope).
+
+---
+
+## References
+
+- **Phase 188.1 Reality**: `docs/development/current/main/phases/phase-188.1/README.md` § Implementation Reality
+- **LoopRegion Definition**: `src/mir/control_form.rs` (lines 40-62)
+- **StepTreeFeatures Definition**: `src/mir/control_tree/step_tree.rs` (lines 17-25)
+- **LoopForm/LoopShape Definition**: `src/mir/loop_form.rs`, `src/mir/control_form.rs`
+
+---
+
+## End of Phase 188.2 Planning Stub
+
+**Status**: Option A is complete; remaining work is Phase 188.3+
+**Dependencies**: Phase 188.1 “Implementation Reality” remains SSOT