feat(joinir): Phase 286 P2.6 - Pattern3 Plan 基盤 + Pattern1 退行修正

## Pattern1 退行修正（構造的 Fail-Fast）
- Router に pattern_kind ガード追加（ctx.pattern_kind != Pattern1SimpleWhile → skip）
- has_if_else_statement() ヘルパー追加（再帰版、ScopeBox/Loop 内もチェック）
- Pattern1 extractor に if-else 拒否追加

## Pattern3 Plan 基盤
- DomainPlan: Pattern3IfPhiPlan 構造体追加
- Extractor: extract_pattern3_plan() 追加
- Normalizer: normalize_pattern3_if_phi() スタブ追加（レガシー JoinIR へフォールバック）
- Router: PLAN_EXTRACTORS に Pattern3 追加

## テスト結果
- quick 154 PASS
- Phase 118 smoke PASS（出力 12、退行解消）

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

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

src/mir/builder/control_flow/plan/mod.rs

@@ -53,6 +53,8 @@ pub(in crate::mir::builder) enum DomainPlan {
     Pattern9AccumConstLoop(Pattern9AccumConstLoopPlan),
     /// Pattern8: Boolean Predicate Scan (Phase 286 P2.4)
     Pattern8BoolPredicateScan(Pattern8BoolPredicateScanPlan),
+    /// Pattern3: Loop with If-Phi (Phase 286 P2.6)
+    Pattern3IfPhi(Pattern3IfPhiPlan),
 }
 
 /// Phase 273 P0: Scan direction for forward/reverse scan
@@ -176,6 +178,53 @@ pub(in crate::mir::builder) struct Pattern8BoolPredicateScanPlan {
     pub step_lit: i64,
 }
 
+/// Phase 286 P2.6: Extracted structure for Pattern3 (Loop with If-Phi)
+///
+/// This structure contains all the information needed to lower an if-phi merge loop.
+/// Pattern3 is a loop with conditional carrier update via if-else branching.
+///
+/// # Structure
+/// ```text
+/// loop(i < N) {
+///     if (condition) {
+///         carrier = then_update
+///     } else {
+///         carrier = else_update
+///     }
+///     i = i + step
+/// }
+/// ```
+///
+/// # CFG Layout
+/// ```text
+/// preheader → header(PHI: i, carrier) → body(if_condition)
+///              ↓                            ↓
+///            after                     then | else
+///                                        ↓     ↓
+///                                       merge(PHI: carrier)
+///                                          ↓
+///                                        step(i_next)
+///                                          ↓
+///                                      back-edge to header
+/// ```
+#[derive(Debug, Clone)]
+pub(in crate::mir::builder) struct Pattern3IfPhiPlan {
+    /// Loop variable name (e.g., "i")
+    pub loop_var: String,
+    /// Carrier variable name (e.g., "sum")
+    pub carrier_var: String,
+    /// Loop condition AST (e.g., `i < 3`)
+    pub condition: ASTNode,
+    /// If condition AST (e.g., `i > 0`)
+    pub if_condition: ASTNode,
+    /// Then branch update AST (e.g., `sum + 1`)
+    pub then_update: ASTNode,
+    /// Else branch update AST (e.g., `sum + 0`)
+    pub else_update: ASTNode,
+    /// Loop increment expression AST (e.g., `i + 1`)
+    pub loop_increment: ASTNode,
+}
+
 // ============================================================================
 // CorePlan (固定語彙 - 構造ノードのみ)
 // ============================================================================

src/mir/builder/control_flow/plan/normalizer.rs

@@ -14,7 +14,7 @@
 use super::{
     CoreEffectPlan, CoreLoopPlan, CorePhiInfo, CorePlan, DomainPlan,
     ScanWithInitPlan, SplitScanPlan, Pattern1SimpleWhilePlan, Pattern9AccumConstLoopPlan,
-    Pattern8BoolPredicateScanPlan,
+    Pattern8BoolPredicateScanPlan, Pattern3IfPhiPlan,
 };
 use crate::mir::builder::control_flow::joinir::patterns::router::LoopPatternContext;
 use crate::mir::builder::MirBuilder;
@@ -44,6 +44,7 @@ impl PlanNormalizer {
             DomainPlan::Pattern1SimpleWhile(parts) => Self::normalize_pattern1_simple_while(builder, parts, ctx),
             DomainPlan::Pattern9AccumConstLoop(parts) => Self::normalize_pattern9_accum_const_loop(builder, parts, ctx),
             DomainPlan::Pattern8BoolPredicateScan(parts) => Self::normalize_pattern8_bool_predicate_scan(builder, parts, ctx),
+            DomainPlan::Pattern3IfPhi(parts) => Self::normalize_pattern3_if_phi(builder, parts, ctx),
         }
     }
 
@@ -2044,4 +2045,35 @@ impl PlanNormalizer {
 
         Ok(CorePlan::Loop(loop_plan))
     }
+
+    /// Phase 286 P2.6: Pattern3 (Loop with If-Phi) → CorePlan 変換
+    ///
+    /// # Status: PoC Stub
+    ///
+    /// This is a minimal stub implementation for Phase 286 P2.6 PoC.
+    /// The extractor works and will be called, but this normalizer
+    /// returns an error to fallback to the legacy Pattern3 implementation.
+    ///
+    /// # CFG Structure (Design - not yet implemented)
+    /// ```text
+    /// preheader → header(PHI: i, carrier) → body(if_condition)
+    ///              ↓                            ↓
+    ///            after                     then | else
+    ///                                        ↓     ↓
+    ///                                       merge(PHI: carrier)
+    ///                                          ↓
+    ///                                        step(i_next)
+    ///                                          ↓
+    ///                                      back-edge to header
+    /// ```
+    fn normalize_pattern3_if_phi(
+        _builder: &mut MirBuilder,
+        _parts: Pattern3IfPhiPlan,
+        _ctx: &LoopPatternContext,
+    ) -> Result<CorePlan, String> {
+        // Phase 286 P2.6 PoC: Stub implementation
+        // Extractor validates Pattern3 structure, but normalization is deferred
+        // Fallback to legacy Pattern3 implementation via Ok(None) in router
+        Err("Pattern3 Plan normalization not yet implemented (PoC stub - fallback to legacy)".to_string())
+    }
 }