feat(joinir): Phase 286 P2.4 - Pattern8 BoolPredicateScan Plan化 PoC

## 概要
Pattern8 (BoolPredicateScan) を Plan extraction routing に追加。
static box 除外（Phase 269 決定）を尊重し、非 static box fixture で PoC。

## 実装内容
- Pattern8BoolPredicateScanPlan struct + DomainPlan variant
- extract_pattern8_plan(): 条件・predicate check・increment 抽出
- normalize_pattern8_bool_predicate_scan(): PoC stub（CoreExitPlan::Return 未統合）
- PLAN_EXTRACTORS テーブルに Pattern8 追加（3rd priority）
- エラーフォールバック: Plan normalization 失敗時 → legacy Pattern8 へ

## 動作フロー
Plan extraction MATCHED → normalization failed (PoC stub) → legacy Pattern8 MATCHED

## 検証結果
- Integration: phase286_pattern8_plan_poc_vm PASS (exit 7)
- Regression: quick 154 PASS, 0 FAILED

🤖 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

@@ -185,6 +185,7 @@ 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)
 /// - Pattern4: Continue loop (more specific than Pattern1)
 /// - Pattern9: Accum const loop (2 carriers, more specific than Pattern1)
 /// - Pattern1: Simple while (fallback)
@@ -197,6 +198,10 @@ static PLAN_EXTRACTORS: &[PlanExtractorEntry] = &[
         name: "Pattern7_SplitScan (Phase 273)",
         extractor: PlanExtractorVariant::WithPostLoop(super::pattern7_split_scan::extract_split_scan_plan),
     },
+    PlanExtractorEntry {
+        name: "Pattern8_BoolPredicateScan (Phase 286 P2.4)",
+        extractor: PlanExtractorVariant::Simple(super::extractors::pattern8::extract_pattern8_plan),
+    },
     PlanExtractorEntry {
         name: "Pattern4_Continue (Phase 286 P2)",
         extractor: PlanExtractorVariant::Simple(super::extractors::pattern4::extract_pattern4_plan),
@@ -238,17 +243,31 @@ fn try_plan_extractors(
             }
         };
 
-        // If extraction succeeded, lower and return
+        // If extraction succeeded, try lowering
         if let Some(domain_plan) = plan_opt {
             let log_msg = format!("route=plan strategy=extract pattern={}", entry.name);
             trace::trace().pattern("route", &log_msg, true);
-            return lower_via_plan(builder, domain_plan, ctx);
-        }
 
-        // Extraction returned None - try next extractor
-        if ctx.debug {
-            let debug_msg = format!("{} extraction returned None, trying next pattern", entry.name);
-            trace::trace().debug("route", &debug_msg);
+            // Phase 286 P2.4: Catch normalization errors for PoC fallback
+            // Pattern8 PoC returns error from normalizer to trigger legacy fallback
+            match lower_via_plan(builder, domain_plan, ctx) {
+                Ok(result) => return Ok(result),
+                Err(e) if e.contains("[normalizer/pattern8]") => {
+                    // Pattern8 PoC: normalization not yet supported, continue to legacy Pattern8
+                    if ctx.debug {
+                        trace::trace().debug("route", &format!("Pattern8 Plan PoC: normalization failed ({}), continuing to legacy Pattern8", e));
+                    }
+                    // Continue to next extractor (or legacy patterns)
+                    continue; // Explicit continue to skip "extraction returned None" log
+                }
+                Err(e) => return Err(e), // Real errors propagate
+            }
+        } else {
+            // Extraction returned None - try next extractor
+            if ctx.debug {
+                let debug_msg = format!("{} extraction returned None, trying next pattern", entry.name);
+                trace::trace().debug("route", &debug_msg);
+            }
         }
     }