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/decls.rs

@@ -75,8 +75,8 @@ impl super::MirBuilder {
                             box_type: "ArrayBox".to_string(),
                             args: vec![],
                         })?;
-                        self.value_origin_newbox.insert(pid, "ArrayBox".to_string());
-                        self.value_types
+                        self.type_ctx.value_origin_newbox.insert(pid, "ArrayBox".to_string());
+                        self.type_ctx.value_types
                             .insert(pid, super::MirType::Box("ArrayBox".to_string()));
                         // Explicitly call birth() to initialize internal state
                         self.emit_instruction(MirInstruction::BoxCall {
@@ -112,7 +112,7 @@ impl super::MirBuilder {
                     self.variable_map.insert(p.clone(), pid);
                     // 関数スコープ SlotRegistry にも登録しておくよ（観測専用）
                     if let Some(reg) = self.current_slot_registry.as_mut() {
-                        let ty = self.value_types.get(&pid).cloned();
+                        let ty = self.type_ctx.value_types.get(&pid).cloned();
                         reg.ensure_slot(p, ty);
                     }
                 }