refactor(plan): Phase 273 P3+ - Legacy code removal

Phase 273 P3+ 完成: レガシーコード削除 + 未使用 import 整理

Removed Legacy Items:
1. emit_scan_with_init_edgecfg() - Pattern6 固有の emission 関数
   - File deleted: src/mir/builder/emission/loop_scan_with_init.rs (~144 lines)
   - Replaced by: generalized Frag API (Phase 273 P2)

2. CoreCarrierInfo struct - Legacy carrier representation
   - Removed from: src/mir/builder/control_flow/plan/mod.rs (~15 lines)
   - Replaced by: CorePhiInfo (generalized PHI representation)

3. verify_carrier() function - CoreCarrierInfo validator
   - Removed from: src/mir/builder/control_flow/plan/verifier.rs (~15 lines)
   - Replaced by: generalized PHI verification (V7-V9)

Code Cleanup:
- cargo fix applied: unused imports removed (~30 files)
- Verifier invariants updated: V1→V2-V9 (carrier→PHI model)
- Module declaration cleanup in emission/mod.rs

Impact:
- Total lines removed: ~174 lines (net reduction)
- Pattern-agnostic architecture strengthened
- All legacy Pattern6 references eliminated

Tests:
- ✅ VM tests PASS (phase254/256/258)
- ✅ LLVM tests PASS (phase256/258)

🤖 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

@@ -192,21 +192,7 @@ pub(in crate::mir::builder) struct CoreLoopPlan {
     pub final_values: Vec<(String, ValueId)>,
 }
 
-/// Phase 273 P1: Loop carrier (PHI variable)
-#[derive(Debug, Clone)]
-pub(in crate::mir::builder) struct CoreCarrierInfo {
-    /// Variable name (for variable_map update)
-    pub name: String,
-
-    /// Initial value (from preheader)
-    pub init_value: ValueId,
-
-    /// Step value (from step block, back-edge)
-    pub step_value: ValueId,
-
-    /// PHI destination (loop variable inside loop)
-    pub phi_dst: ValueId,
-}
+// Phase 273 P3: CoreCarrierInfo removed (replaced by CorePhiInfo)
 
 /// Phase 273 P1: Conditional plan
 #[derive(Debug, Clone)]