fix(pattern2): abort entire Pattern2 on unpromoted LoopBodyLocal instead of partial execution

Phase 263 P0: Pattern2 で処理できない LoopBodyLocal を検出したら Pattern2 全体を早期終了

Changes:
- promote_step_box.rs: 戻り値を Result<Option<_>, String> に変更
  - Reject を二分化: 対象外（not_readonly等）→ Ok(None)、対象だが未対応 → Err
- pattern2_lowering_orchestrator.rs: Ok(None) 検出で早期 return
- apps/tests/phase263_p0_pattern2_seg_min.hako: test simplification

後続経路（legacy binding等）へ fallback させる（detection→extract→lower SSOT 維持）
Fail-Fast 原則: 対象外は Ok(None) で後続経路へ、対象だが未対応は Err で即座に失敗

Fixes: core_direct_array_oob_set_rc_vm smoke test FAIL

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

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

apps/tests/phase263_p0_pattern2_seg_min.hako

@@ -7,29 +7,19 @@
 
 static box Main {
     main() {
-        local result = parse_segments()
+        local i = 0
+        local seg = ""
+
+        loop(i < 5) {
+            seg = "segment"  // ← loop body で代入（ReadOnlySlot 契約違反）
+
+            if seg == "end" {
+                break
+            }
+
+            i = i + 1
+        }
+
         return 0
     }
 }
-
-// Pattern2 が検出するループ構造
-method parse_segments() {
-    local i = 0
-    local seg = ""
-
-    loop(i < 5) {
-        seg = get_segment(i)  // ← loop body で代入（ReadOnlySlot 契約違反）
-
-        if seg == "end" {
-            break
-        }
-
-        i = i + 1
-    }
-
-    return i
-}
-
-method get_segment(idx) {
-    return "seg"
-}

src/mir/builder/control_flow/joinir/patterns/pattern2_lowering_orchestrator.rs

@@ -63,7 +63,15 @@ impl Pattern2LoweringOrchestrator {
         let inputs = ApplyPolicyStepBox::apply(condition, body, facts)?;
 
         let promoted = PromoteStepBox::run(builder, condition, body, inputs, debug, verbose)?;
-        let mut inputs = promoted.inputs;
+        let mut inputs = match promoted {
+            Some(result) => result.inputs,
+            None => {
+                // Phase 263 P0: Pattern2 cannot handle this loop (e.g., reassigned LoopBodyLocal)
+                // Return Ok(None) to allow router to try next path (legacy binding)
+                super::super::trace::trace().debug("pattern2", "Pattern2 aborted (promotion failed), allowing fallback");
+                return Ok(None);
+            }
+        };
 
         // Phase 256.5: Wire current_static_box_name from builder context or function name
         inputs.current_static_box_name = current_box_name_for_lowering(builder);

src/mir/builder/control_flow/joinir/patterns/pattern2_steps/promote_step_box.rs

@@ -26,9 +26,11 @@ impl PromoteStepBox {
         mut inputs: Pattern2Inputs,
         debug: bool,
         verbose: bool,
-    ) -> Result<PromoteStepResult, String> {
-        Self::promote_and_prepare_carriers(builder, condition, body, &mut inputs, debug, verbose)?;
-        Ok(PromoteStepResult { inputs })
+    ) -> Result<Option<PromoteStepResult>, String> {
+        match Self::promote_and_prepare_carriers(builder, condition, body, &mut inputs, debug, verbose)? {
+            Some(()) => Ok(Some(PromoteStepResult { inputs })),
+            None => Ok(None),  // Pattern2 cannot handle this loop
+        }
     }
 
     pub(in crate::mir::builder) fn promote_and_prepare_carriers(
@@ -38,7 +40,7 @@ impl PromoteStepBox {
         inputs: &mut Pattern2Inputs,
         debug: bool,
         verbose: bool,
-    ) -> Result<(), String> {
+    ) -> Result<Option<()>, String> {
         use crate::mir::join_ir::lowering::digitpos_condition_normalizer::DigitPosConditionNormalizer;
         use crate::mir::loop_pattern_detection::loop_condition_scope::LoopConditionScopeBox;
 
@@ -119,10 +121,26 @@ impl PromoteStepBox {
                         inputs.read_only_body_local_slot = Some(slot);
                     }
                     PolicyDecision::Reject(reason) => {
-                        return Err(error_messages::format_error_pattern2_promotion_failed(
-                            &cond_body_local_vars,
-                            &reason,
-                        ));
+                        // Phase 263 P0 + Step 2.5: Reject を二分化
+                        if reason.contains("not_readonly")
+                            || reason.contains("No promotable pattern detected")
+                        {
+                            // 対象外: Pattern2 で処理できない形 → Ok(None) で後続経路へ
+                            #[cfg(debug_assertions)]
+                            {
+                                eprintln!(
+                                    "[pattern2/promote_step] Pattern2 対象外（LoopBodyLocal {:?}）: {}. 後続経路へfallback.",
+                                    cond_body_local_vars, reason
+                                );
+                            }
+                            return Ok(None);  // Pattern2 全体を中止
+                        } else {
+                            // 対象だが未対応（freeze級）: 実装バグ or 将来実装予定 → Err で Fail-Fast
+                            return Err(format!(
+                                "[pattern2/promote_step] Pattern2 未対応エラー（LoopBodyLocal {:?}）: {}",
+                                cond_body_local_vars, reason
+                            ));
+                        }
                     }
                     PolicyDecision::None => {}
                 }
@@ -174,7 +192,7 @@ impl PromoteStepBox {
             }
         }
 
-        Ok(())
+        Ok(Some(()))
     }
 
 }