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/joinir/patterns/router.rs

@@ -190,6 +190,7 @@ enum PlanExtractorVariant {
 /// Order is important: more specific patterns first
 /// - Pattern6/7: Need fn_body for capture analysis
 /// - Pattern8: Bool predicate scan (early-exit return, more specific)
+/// - Pattern3: If-phi merge (carrier with conditional update, more specific than Pattern4)
 /// - Pattern4: Continue loop (more specific than Pattern1)
 /// - Pattern9: Accum const loop (2 carriers, more specific than Pattern1)
 /// - Pattern1: Simple while (fallback)
@@ -206,6 +207,10 @@ static PLAN_EXTRACTORS: &[PlanExtractorEntry] = &[
         name: "Pattern8_BoolPredicateScan (Phase 286 P2.4)",
         extractor: PlanExtractorVariant::Simple(super::extractors::pattern8::extract_pattern8_plan),
     },
+    PlanExtractorEntry {
+        name: "Pattern3_IfPhi (Phase 286 P2.6)",
+        extractor: PlanExtractorVariant::Simple(super::extractors::pattern3::extract_pattern3_plan),
+    },
     PlanExtractorEntry {
         name: "Pattern4_Continue (Phase 286 P2)",
         extractor: PlanExtractorVariant::Simple(super::extractors::pattern4::extract_pattern4_plan),
@@ -232,6 +237,16 @@ fn try_plan_extractors(
     use super::super::trace;
 
     for entry in PLAN_EXTRACTORS {
+        // Phase 286 P2.6: Pattern1 Plan guard (structural Fail-Fast)
+        // Pattern1 should only match Pattern1SimpleWhile pattern_kind
+        // This prevents Pattern1 from incorrectly matching Pattern3 fixtures
+        if entry.name.contains("Pattern1") {
+            use crate::mir::loop_pattern_detection::LoopPatternKind;
+            if ctx.pattern_kind != LoopPatternKind::Pattern1SimpleWhile {
+                continue;
+            }
+        }
+
         // Try extraction based on variant
         let plan_opt = match &entry.extractor {
             PlanExtractorVariant::Simple(extractor) => {
@@ -266,6 +281,23 @@ fn try_plan_extractors(
             let log_msg = format!("route=plan strategy=extract pattern={}", entry.name);
             trace::trace().pattern("route", &log_msg, true);
 
+            // Phase 286 P2.6: Pattern3 PoC exception - allow fallback to legacy
+            // Pattern3 extractor validates structure, but normalizer is stub
+            if entry.name.contains("Pattern3") {
+                match lower_via_plan(builder, domain_plan, ctx) {
+                    Ok(value_id) => return Ok(value_id),
+                    Err(_) => {
+                        if ctx.debug {
+                            trace::trace().debug(
+                                "route/plan",
+                                "Pattern3 Plan normalization stub - fallback to legacy Pattern3",
+                            );
+                        }
+                        continue; // Try next extractor (will eventually reach legacy Pattern3)
+                    }
+                }
+            }
+
             // Phase 286 P2.4.1: Fail-Fast - extract 成功 → normalize/lower 失敗は即 Err
             // 構造的 Fail-Fast: 文字列判定なし、extract が Some なら fallback 禁止
             return lower_via_plan(builder, domain_plan, ctx);