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/joinir/merge/instruction_rewriter.rs

@@ -18,8 +18,7 @@ use crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper;
 use crate::mir::join_ir::lowering::error_tags;
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
 // Phase 256 P1.7: Removed join_func_name import - no longer needed
-use crate::mir::types::ConstValue;
-use crate::mir::{BasicBlock, BasicBlockId, MirFunction, MirInstruction, MirModule, MirType, ValueId};
+use crate::mir::{BasicBlock, BasicBlockId, MirInstruction, MirModule, ValueId};
 use std::collections::BTreeMap; // Phase 222.5-E: HashMap → BTreeMap for determinism
 use std::collections::BTreeSet;
 

src/mir/builder/control_flow/joinir/merge/rewriter/mod.rs

@@ -39,5 +39,5 @@ pub(super) mod terminator; // Phase 260 P0.1 Step 5: Terminator remapping extrac
 pub(super) mod type_propagation; // Phase 260 P0.1 Step 4: Type propagation extracted ✅
 
 // Re-export public API
-pub(super) use helpers::is_skippable_continuation; // Phase 260 P0.1 Step 3: From helpers ✅
+ // Phase 260 P0.1 Step 3: From helpers ✅
 pub(super) use super::instruction_rewriter::merge_and_rewrite; // Still in parent (TODO: extract)

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

@@ -36,7 +36,6 @@ use super::pattern2_steps::gather_facts_step_box::GatherFactsStepBox;
 use super::pattern2_steps::merge_step_box::MergeStepBox;
 use super::pattern2_steps::normalize_body_step_box::NormalizeBodyStepBox;
 use super::pattern2_steps::post_loop_early_return_step_box::PostLoopEarlyReturnStepBox;
-use super::pattern2_steps::promote_step_box::PromoteStepBox;
 
 pub(crate) struct Pattern2LoweringOrchestrator;
 

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

@@ -4,7 +4,6 @@
 
 use crate::ast::ASTNode;
 use crate::mir::builder::MirBuilder;
-use crate::mir::ValueId;
 
 use super::super::pattern2_inputs_facts_box::{Pattern2DebugLog, Pattern2Inputs};
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;

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

@@ -10,7 +10,7 @@
 use crate::ast::ASTNode;
 use crate::mir::builder::MirBuilder;
 
-use super::super::pattern2::api::{try_promote, PromoteDecision, PromoteStepResult};
+use super::super::pattern2::api::{try_promote, PromoteDecision};
 use super::super::pattern2_inputs_facts_box::Pattern2Inputs;
 
 pub(crate) struct PromoteStepBox;

src/mir/builder/control_flow/joinir/patterns/pattern6_scan_with_init.rs

@@ -33,11 +33,8 @@
 //! - MethodCall allowed_in_condition() = false, but allowed_in_init() = true
 //! - Need to hoist MethodCall to init phase
 
-use super::super::trace;
-use super::common::var; // Phase 255 P2: Use shared var() helper
-use crate::ast::{ASTNode, BinaryOperator, LiteralValue};
-use crate::mir::builder::MirBuilder;
-use crate::mir::ValueId;
+ // Phase 255 P2: Use shared var() helper
+use crate::ast::ASTNode;
 
 // Phase 273 P1: Import DomainPlan types (Plan renamed to DomainPlan)
 use crate::mir::builder::control_flow::plan::{DomainPlan, ScanDirection as PlanScanDirection, ScanWithInitPlan};

src/mir/builder/control_flow/joinir/patterns/pattern8_scan_bool_predicate.rs

@@ -34,7 +34,6 @@
 //! - Pattern 8: Returns boolean (true/false)
 
 use super::super::trace;
-use super::common::var;
 use crate::ast::{ASTNode, BinaryOperator, LiteralValue, UnaryOperator};
 use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;