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/exprs_lambda.rs

@@ -170,7 +170,7 @@ impl super::MirBuilder {
             captures,
             me,
         })?;
-        self.value_types
+        self.type_ctx.value_types
             .insert(dst, crate::mir::MirType::Box("FunctionBox".to_string()));
         Ok(dst)
     }