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/verification/cfg.rs

@@ -1,7 +1,7 @@
 use crate::mir::function::MirFunction;
 use crate::mir::verification::utils;
 use crate::mir::verification_types::VerificationError;
-use crate::mir::{BasicBlockId, MirInstruction, ValueId};
+use crate::mir::{BasicBlockId, ValueId};
 use std::collections::{HashMap, HashSet};
 
 /// Verify CFG references and reachability