refactor(extractors): Phase 282 P9a - CommonExtractionHelpers SSOT統合（スコープ限定版）

# Phase 282 P9a 完了 (Scope-Limited Integration)

## 実装内容
- **common_helpers.rs 作成**: 4グループの共通ヘルパー統合 (316行)
  - Group 1: Control Flow Counting (count_control_flow - 汎用カウンター)
  - Group 2: Control Flow Detection (has_break/continue/return_statement)
  - Group 3: Condition Validation (extract_loop_variable, is_true_literal)
  - Group 4: Pattern5専用ヘルパー (validate_continue_at_end, validate_break_in_simple_if)

- **Pattern統合完了**: Pattern5 → Pattern4 → Pattern2 → Pattern1
  - Pattern5: ~90行削減 (5 tests PASS)
  - Pattern4: ~66行削減 (5 tests PASS)
  - Pattern2: ~67行削減 (4 tests PASS)
  - Pattern1: ~28行削減 (3 tests PASS)
  - Pattern3: 別フェーズに延期（pattern固有ロジック除外）

## 成果
- **コード削減**: ~251行（Pattern3除く、total ~400行見込み）
- **テスト**: 40 unit tests PASS (23 common_helpers + 17 extractors)
- **スモークテスト**: 45 PASS, 1 pre-existing FAIL（退行ゼロ）
- **ビルド警告**: 130 → 120 (-10)

## USER CORRECTIONS適用済み
1. ✅ スコープ限定（共通ロジックのみ、pattern固有除外）
2. ✅ Placeholder禁止（SSOT違反排除）
3. ✅ 統合順序変更（Pattern3を最後/別フェーズへ）

## 追加ドキュメント
- Phase 284 計画追加（Return as ExitKind SSOT）
- 10-Now.md, 30-Backlog.md 更新

🎯 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

@@ -309,7 +309,7 @@ pub(crate) fn route_loop_pattern(
     )? {
         Some(domain_plan) => {
             // DomainPlan extracted successfully
-            trace::trace().pattern("route", "route=plan pattern=Pattern6_ScanWithInit (Phase 273)", true);
+            trace::trace().pattern("route", "route=plan strategy=extract pattern=Pattern6_ScanWithInit (Phase 273)", true);
 
             // Step 1: Normalize DomainPlan → CorePlan
             let core_plan = PlanNormalizer::normalize(builder, domain_plan, ctx)?;
@@ -340,7 +340,7 @@ pub(crate) fn route_loop_pattern(
     )? {
         Some(domain_plan) => {
             // DomainPlan extracted successfully
-            trace::trace().pattern("route", "route=plan pattern=Pattern7_SplitScan (Phase 273)", true);
+            trace::trace().pattern("route", "route=plan strategy=extract pattern=Pattern7_SplitScan (Phase 273)", true);
 
             // Step 1: Normalize DomainPlan → CorePlan
             let core_plan = PlanNormalizer::normalize(builder, domain_plan, ctx)?;
@@ -370,7 +370,7 @@ pub(crate) fn route_loop_pattern(
     // Phase 273 P0.1: Pattern6 skip logic removed (entry no longer in LOOP_PATTERNS)
     for entry in LOOP_PATTERNS {
         if (entry.detect)(builder, ctx) {
-            let log_msg = format!("route=joinir pattern={} (Phase 194+)", entry.name);
+            let log_msg = format!("route=joinir strategy=extract pattern={} (Phase 194+)", entry.name);
             trace::trace().pattern("route", &log_msg, true);
             return (entry.lower)(builder, ctx);
         }