feat(joinir): Phase 212.5 Structural if detection for Pattern 3 routing

Phase 212.5 で発見した「ループ内 if が Pattern 1 に誤ルーティング」問題を修正。
構造ベース if 検出により、単一キャリアの if-update パターンも Pattern 3 へ正しくルーティング可能に。

## Changes

### 1. AST Feature Extractor (ast_feature_extractor.rs)
- **Added**: `detect_if_in_body()` function
  - Detects ANY if statement in loop body (not just if-else)
  - Enables structural if detection vs carrier-count heuristic
- **Modified**: `extract_features()`
  - Uses `detect_if_in_body()` for `has_if` detection
  - Removes dependency on carrier count for if detection

### 2. Loop Pattern Classification (loop_pattern_detection/mod.rs)
- **Modified**: `classify()` function
  - Pattern 3: `carrier_count > 1` → `has_if && carrier_count >= 1`
  - Pattern 1: `!has_if_else_phi` → `!has_if`
  - Now routes single-carrier if-update patterns to Pattern 3

## Verification

Test case: `apps/tests/phase212_if_sum_min.hako`

### Before (Phase 212):
- ❌ Routed to Pattern 1 (wrong)
- ❌ if statement disappeared in MIR
- ❌ Carriers: only `i` (sum missing)

### After (Phase 212.5):
- ✅ Routed to Pattern 3 (correct!)
- ✅ MIR contains PHI nodes: `%31 = phi [%25, bb9], [%29, bb10]`
- ✅ Carriers: `i`, `sum`, `count` detected

Pattern routing log:
```
[joinir/pattern3] Generated JoinIR for Loop with If-Else PHI
[joinir/pattern3] Carriers: i (counter), sum (accumulator), count (counter)
```

## Known Limitation

Pattern 3 lowerer (`lower_loop_with_if_phi_pattern`) is currently a
test-only hardcoded implementation:
- Loop condition: `i <= 5` (hardcoded)
- If condition: `i % 2 == 1` (hardcoded)
- Update logic: `sum + i` (hardcoded)

This is why `phase212_if_sum_min.hako` produces RC=0 instead of RC=2.
Pattern routing is complete; AST-based lowerer generalization is Phase 213.

## Documentation

- `docs/development/current/main/phase212-5-implementation-complete.md`
  - Complete implementation report with verification details

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

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

src/mir/loop_pattern_detection/mod.rs

@@ -240,18 +240,20 @@ pub(crate) fn extract_features(loop_form: &LoopForm, scope: Option<&LoopScopeSha
 /// structure-based rules. It does NOT depend on function names or
 /// variable names like "sum".
 ///
-/// # Pattern Classification Rules
+/// # Pattern Classification Rules (Phase 212.5: Structural if detection)
 ///
 /// 1. **Pattern 4 (Continue)**: `has_continue == true`
 ///    - Priority: Check first (most specific)
 ///
-/// 2. **Pattern 3 (If-Else PHI)**: `has_if_else_phi && !has_break && !has_continue`
-///    - Multiple carriers indicate if-else with PHI
+/// 2. **Pattern 3 (If-PHI)**: `has_if && carrier_count >= 1 && !has_break && !has_continue`
+///    - Phase 212.5: Changed from carrier_count > 1 to structural if detection
+///    - Includes single-carrier if-update patterns (e.g., if-sum with 1 carrier)
 ///
 /// 3. **Pattern 2 (Break)**: `has_break && !has_continue`
 ///    - Has break but no continue
 ///
-/// 4. **Pattern 1 (Simple While)**: `!has_break && !has_continue && !has_if_else_phi`
+/// 4. **Pattern 1 (Simple While)**: `!has_break && !has_continue && !has_if`
+///    - Phase 212.5: Exclude loops with if statements
 ///    - No control flow alterations
 ///
 /// # Arguments
@@ -271,8 +273,9 @@ pub fn classify(features: &LoopFeatures) -> LoopPatternKind {
         return LoopPatternKind::Pattern4Continue;
     }
 
-    // Pattern 3: If-Else PHI (check before Pattern 1)
-    if features.has_if_else_phi && !features.has_break && !features.has_continue {
+    // Pattern 3: If-PHI (check before Pattern 1)
+    // Phase 212.5: Structural if detection - route to P3 if has_if && carrier_count >= 1
+    if features.has_if && features.carrier_count >= 1 && !features.has_break && !features.has_continue {
         return LoopPatternKind::Pattern3IfPhi;
     }
 
@@ -282,7 +285,8 @@ pub fn classify(features: &LoopFeatures) -> LoopPatternKind {
     }
 
     // Pattern 1: Simple While
-    if !features.has_break && !features.has_continue && !features.has_if_else_phi {
+    // Phase 212.5: Exclude loops with if statements (they go to P3)
+    if !features.has_break && !features.has_continue && !features.has_if {
         return LoopPatternKind::Pattern1SimpleWhile;
     }