refactor(mir): Phase 137-6-S1 - choose_pattern_kind SSOT入口を新設

## 目的
Pattern 選択ロジックを SSOT 化し、将来の Canonicalizer 委譲に備える

## 変更内容

### 新規関数: `choose_pattern_kind()`
- 場所: `src/mir/builder/control_flow/joinir/routing.rs`
- 責務: Pattern 選択の SSOT 入口
- 実装: 既存の LoopFeatures ベース選択ロジックを集約

### LoopPatternContext の更新
- `new()` で `choose_pattern_kind()` を使用
- 既存の分散した選択ロジックを SSOT に統一

## 効果
- ✅ Pattern 選択ロジックの SSOT 化（1箇所に集約）
- ✅ 将来の Canonicalizer 委譲に備えた構造確立
- ✅ 既定挙動完全不変（既存テスト全て PASS）

## テスト結果
- ✅ `cargo build --release`: 成功
- ✅ スモークテスト（simple_*）: 5/5 PASS
- ✅ 退行なし

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

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

src/mir/builder/control_flow/joinir/loop_context.rs

@@ -88,11 +88,7 @@ impl<'a> LoopProcessingContext<'a> {
     ///
     /// This should be called after running the canonicalizer.
     /// After this call, `verify_parity()` can be used to check consistency.
-    pub fn set_canonicalizer_result(
-        &mut self,
-        skeleton: LoopSkeleton,
-        decision: RoutingDecision,
-    ) {
+    pub fn set_canonicalizer_result(&mut self, skeleton: LoopSkeleton, decision: RoutingDecision) {
         self.skeleton = Some(skeleton);
         self.decision = Some(decision);
     }
@@ -304,8 +300,10 @@ mod tests {
         let mut ctx = make_simple_context(&condition, &body);
 
         // Set canonicalizer result with fail-fast decision
-        let decision =
-            RoutingDecision::fail_fast(vec![CapabilityTag::ConstStep], "Test fail-fast".to_string());
+        let decision = RoutingDecision::fail_fast(
+            vec![CapabilityTag::ConstStep],
+            "Test fail-fast".to_string(),
+        );
         ctx.set_canonicalizer_result(
             LoopSkeleton {
                 steps: vec![],