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/utils.rs

@@ -185,7 +185,7 @@ impl super::MirBuilder {
         use crate::runtime::{CoreBoxId, CoreMethodId};
 
         // 1. box_val の型を取得
-        let box_ty = self.value_types.get(&box_val)?;
+        let box_ty = self.type_ctx.value_types.get(&box_val)?;
 
         // 2. Box 型名を取得
         let box_name = match box_ty {
@@ -255,9 +255,9 @@ impl super::MirBuilder {
         // Check environment variable for unified call usage, with safe overrides for core/user boxes
         let use_unified_env = super::calls::call_unified::is_unified_call_enabled();
         // First, try to determine the box type
-        let mut box_type: Option<String> = self.value_origin_newbox.get(&box_val).cloned();
+        let mut box_type: Option<String> = self.type_ctx.value_origin_newbox.get(&box_val).cloned();
         if box_type.is_none() {
-            if let Some(t) = self.value_types.get(&box_val) {
+            if let Some(t) = self.type_ctx.value_types.get(&box_val) {
                 match t {
                     super::MirType::String => box_type = Some("StringBox".to_string()),
                     super::MirType::Box(name) => box_type = Some(name.clone()),
@@ -316,9 +316,9 @@ impl super::MirBuilder {
             effects,
         })?;
         if let Some(d) = dst {
-            let mut recv_box: Option<String> = self.value_origin_newbox.get(&box_val).cloned();
+            let mut recv_box: Option<String> = self.type_ctx.value_origin_newbox.get(&box_val).cloned();
             if recv_box.is_none() {
-                if let Some(t) = self.value_types.get(&box_val) {
+                if let Some(t) = self.type_ctx.value_types.get(&box_val) {
                     match t {
                         super::MirType::String => recv_box = Some("StringBox".to_string()),
                         super::MirType::Box(name) => recv_box = Some(name.clone()),
@@ -328,12 +328,12 @@ impl super::MirBuilder {
             }
             if let Some(bt) = recv_box {
                 if let Some(mt) = self.plugin_method_sigs.get(&(bt.clone(), method.clone())) {
-                    self.value_types.insert(d, mt.clone());
+                    self.type_ctx.value_types.insert(d, mt.clone());
                 } else {
                     // Phase 84-4-B: ビルトイン Box のメソッド戻り値型推論
                     // plugin_method_sigs に登録されていない場合のフォールバック
                     if let Some(ret_ty) = self.infer_boxcall_return_type(box_val, &method) {
-                        self.value_types.insert(d, ret_ty.clone());
+                        self.type_ctx.value_types.insert(d, ret_ty.clone());
 
                         if std::env::var("NYASH_BOXCALL_TYPE_TRACE").ok().as_deref() == Some("1") {
                             eprintln!(