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/observe/ssa.rs

@@ -7,8 +7,8 @@ pub(crate) fn emit_phi(builder: &MirBuilder, dst: ValueId, inputs: &Vec<(BasicBl
     let preds: Vec<serde_json::Value> = inputs
         .iter()
         .map(|(bb, v)| {
-            let t = builder.value_types.get(v).cloned();
-            let o = builder.value_origin_newbox.get(v).cloned();
+            let t = builder.type_ctx.value_types.get(v).cloned();
+            let o = builder.type_ctx.value_origin_newbox.get(v).cloned();
             serde_json::json!({
                 "bb": bb.0,
                 "v": v.0,
@@ -18,13 +18,13 @@ pub(crate) fn emit_phi(builder: &MirBuilder, dst: ValueId, inputs: &Vec<(BasicBl
         })
         .collect();
     let decided_t = builder
-        .value_types
+        .type_ctx.value_types
         .get(&dst)
         .cloned()
         .map(|tt| format!("{:?}", tt))
         .unwrap_or_default();
     let decided_o = builder
-        .value_origin_newbox
+        .type_ctx.value_origin_newbox
         .get(&dst)
         .cloned()
         .unwrap_or_default();