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>

src/mir/builder/control_flow/joinir/patterns/router.rs

@@ -74,6 +74,11 @@ pub(crate) struct LoopPatternContext<'a> {
     /// SSOT Principle: Avoid re-detecting ConditionalStep in lowering phase.
     #[allow(dead_code)]
     pub skeleton: Option<&'a LoopSkeleton>,
+
+    /// Phase 188.3: Cached StepTree max_loop_depth for Pattern6
+    /// None if not computed, Some(depth) if Pattern6 candidate
+    /// Avoids re-building StepTree in lowering phase
+    pub step_tree_max_loop_depth: Option<u32>,
 }
 
 impl<'a> LoopPatternContext<'a> {
@@ -111,6 +116,7 @@ impl<'a> LoopPatternContext<'a> {
             pattern_kind,
             fn_body: None, // Phase 200-C: Default to None
             skeleton: None, // Phase 92 P0-2: Default to None
+            step_tree_max_loop_depth: None, // Phase 188.3: Default to None
         }
     }
 
@@ -386,6 +392,11 @@ pub(crate) struct LoopPatternEntry {
 /// Note: func_name is now only used for debug logging, not pattern detection
 /// Phase 286: Pattern5 removed (migrated to Plan-based routing)
 pub(crate) static LOOP_PATTERNS: &[LoopPatternEntry] = &[
+    LoopPatternEntry {
+        name: "Pattern6_NestedLoopMinimal",  // Phase 188.3: 1-level nested loop
+        detect: super::pattern6_nested_minimal::can_lower,
+        lower: super::pattern6_nested_minimal::lower,
+    },
     LoopPatternEntry {
         name: "Pattern4_WithContinue",
         detect: super::pattern4_with_continue::can_lower,