feat(joinir): Phase 286 P3.1 - Pattern2 Plan line 完走（after_bb PHI）

Pattern2（Loop with Break）を Plan/Frag SSOT へ移行。

主な変更:
- Pattern2BreakPlan 追加（DomainPlan variant）
- extract_pattern2_plan() 実装（PoC サブセット厳守）
- normalize_pattern2_break() 実装（6-block CFG, 3 PHI）
- after_bb PHI が本質: carrier_out = PHI(header: carrier_current, break_then: carrier_break)
- router に Pattern2 追加（Pattern1 より前、より具体的）

テスト:
- Fixture B (break without update): PASS (出力 11)
- quick smoke: 154/154 PASS

🤖 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/patterns/router.rs

@@ -219,6 +219,10 @@ static PLAN_EXTRACTORS: &[PlanExtractorEntry] = &[
         name: "Pattern9_AccumConstLoop (Phase 286 P2.3)",
         extractor: PlanExtractorVariant::Simple(super::extractors::pattern9::extract_pattern9_plan),
     },
+    PlanExtractorEntry {
+        name: "Pattern2_Break (Phase 286 P3.1)",
+        extractor: PlanExtractorVariant::Simple(super::extractors::pattern2::extract_pattern2_plan),
+    },
     PlanExtractorEntry {
         name: "Pattern1_SimpleWhile (Phase 286 P2.1)",
         extractor: PlanExtractorVariant::Simple(super::extractors::pattern1::extract_pattern1_plan),