refactor(mir): Remove ScopeContext legacy fields (Phase 2-4/7)

完全移行→削除の安全順序（Option C）に従い、ScopeContextの
deprecated フィールドと sync helpers を完全削除。

## Changes
- Migrated all access sites to scope_ctx.*
- Removed 7 deprecated fields:
  - current_function
  - lexical_scope_stack
  - function_param_names
  - loop_header_stack
  - loop_exit_stack
  - if_merge_stack
  - debug_scope_stack
- Removed 2 sync helpers (sync_scope_ctx_to_legacy, sync_legacy_to_scope_ctx)
- Updated 20+ files with direct field access

## Tests
- cargo build --release: PASS ✅
- Deprecation warnings: 255 → 166 (-89, -35%)

Phase 2 Progress: 4/7 contexts complete (57%)

src/mir/builder/control_flow/joinir/merge/exit_phi_builder.rs

@@ -26,7 +26,7 @@ pub(super) fn build_exit_phi(
 ) -> Result<(Option<ValueId>, BTreeMap<String, ValueId>), String> {
     let mut carrier_phis: BTreeMap<String, ValueId> = BTreeMap::new();
 
-    let exit_phi_result_id = if let Some(ref mut func) = builder.current_function {
+    let exit_phi_result_id = if let Some(ref mut func) = builder.scope_ctx.current_function {
         let mut exit_block = BasicBlock::new(exit_block_id);
 
         // Phase 189-Fix: If we collected return values, create a PHI in exit block

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -149,7 +149,7 @@ pub(super) fn merge_and_rewrite(
 
             // Phase 195 FIX: Reuse existing block if present (preserves PHI from JoinIR Select lowering)
             // ultrathink "finalizer集約案": Don't overwrite blocks with BasicBlock::new()
-            let mut new_block = if let Some(ref mut current_func) = builder.current_function {
+            let mut new_block = if let Some(ref mut current_func) = builder.scope_ctx.current_function {
                 current_func
                     .blocks
                     .remove(&new_block_id)
@@ -811,7 +811,7 @@ pub(super) fn merge_and_rewrite(
             }
 
             // Add block to current function
-            if let Some(ref mut current_func) = builder.current_function {
+            if let Some(ref mut current_func) = builder.scope_ctx.current_function {
                 current_func.add_block(new_block);
             }
         }
@@ -866,7 +866,7 @@ pub(super) fn merge_and_rewrite(
 
         // Use BoundaryInjector to inject Copy instructions
         // Phase 177-3 Option B: Returns reallocations map for condition_bindings with PHI collisions
-        if let Some(ref mut current_func) = builder.current_function {
+        if let Some(ref mut current_func) = builder.scope_ctx.current_function {
             let _reallocations = BoundaryInjector::inject_boundary_copies(
                 current_func,
                 entry_block_remapped,

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

@@ -733,7 +733,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
     #[cfg(debug_assertions)]
     {
         if let Some(boundary) = boundary {
-            if let Some(ref func) = builder.current_function {
+            if let Some(ref func) = builder.scope_ctx.current_function {
                 verify_joinir_contracts(
                     func,
                     entry_block,

src/mir/builder/control_flow/mod.rs

@@ -115,7 +115,7 @@ impl super::MirBuilder {
             "Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder has been removed.\n\
              Function: {}\n\
              Hint: This loop pattern is not supported. All loops must use JoinIR lowering.",
-            self.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
+            self.scope_ctx.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
         )));
     }