refactor(mir): Remove TypeContext legacy fields (Phase 2-3/7)

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

⚠️ 危険ゾーン: TypeFactsBox 等の同名フィールドと混同しないよう、
ファイル単位で手作業移行を実施。

## Changes
- Migrated all MirBuilder access sites to type_ctx.* (manual, 40+ files)
- Removed 3 deprecated fields (value_types, value_kinds, value_origin_newbox)
- Removed 2 sync helpers (sync_type_ctx_to_legacy, sync_legacy_to_type_ctx)
- Verified TypeFactsBox, CalleeGuardBox unchanged (no false positives)

## Tests
- cargo test --release --lib: 1029/1033 PASS
- TypeFactsBox integration: PASS (borrowed references unchanged)
- Deprecation warnings: 456 → 255 (-201, -44%)

## Safety Verification
✅ TypeFactsBox unchanged (still uses &'a BTreeMap borrowed references)
✅ CalleeGuardBox unchanged
✅ CalleeResolverBox unchanged
✅ BoxCompilationContext unchanged

Phase 2 Progress: 3/7 contexts complete (43%)
- ✅ MetadataContext
- ✅ CoreContext
- ✅ TypeContext (this commit)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/stmts.rs

@@ -341,7 +341,7 @@ impl super::MirBuilder {
             self.declare_local_in_current_scope(var_name, var_id)?;
             // SlotRegistry にもローカル変数スロットを登録しておくよ（観測専用）
             if let Some(reg) = self.current_slot_registry.as_mut() {
-                let ty = self.value_types.get(&var_id).cloned();
+                let ty = self.type_ctx.value_types.get(&var_id).cloned();
                 reg.ensure_slot(&var_name, ty);
             }
             last_value = Some(var_id);
@@ -427,7 +427,7 @@ impl super::MirBuilder {
             })?;
             // Future spawn returns a Future<T>; the inner type is not statically known here.
             // Register at least Future<Unknown> to avoid later fail-fast type inference panics.
-            self.value_types
+            self.type_ctx.value_types
                 .insert(future_id, MirType::Future(Box::new(MirType::Unknown)));
             self.variable_map.insert(variable.clone(), future_id);
             if let Some(reg) = self.current_slot_registry.as_mut() {
@@ -442,11 +442,11 @@ impl super::MirBuilder {
             value: expression_value,
         })?;
         let inner = self
-            .value_types
+            .type_ctx.value_types
             .get(&expression_value)
             .cloned()
             .unwrap_or(MirType::Unknown);
-        self.value_types
+        self.type_ctx.value_types
             .insert(future_id, MirType::Future(Box::new(inner)));
         self.variable_map.insert(variable.clone(), future_id);
         if let Some(reg) = self.current_slot_registry.as_mut() {
@@ -467,11 +467,11 @@ impl super::MirBuilder {
             dst: result_id,
             future: future_value,
         })?;
-        let result_type = match self.value_types.get(&future_value) {
+        let result_type = match self.type_ctx.value_types.get(&future_value) {
             Some(MirType::Future(inner)) => (**inner).clone(),
             _ => MirType::Unknown,
         };
-        self.value_types.insert(result_id, result_type);
+        self.type_ctx.value_types.insert(result_id, result_type);
         self.emit_instruction(MirInstruction::Safepoint)?;
         Ok(result_id)
     }