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/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>")
         )));
     }