refactor(phase27.13): Replace @ notation with local variables in UsingResolverBox

- Replace 3 instances of @ notation in _read_file method:
  - @fb → local fb
  - @ok → local ok
  - @content → local content

Result:
- ✅ Build success: cargo build --release (warnings only)
- ✅ Tests: 2/2 existing tests pass (type_sanity, empty_module_returns_none)
- ⚠️ auto_lowering test still blocked by 'using' keyword parser error (separate issue)

Phase 27.13 JoinIR lowering implementation is complete.
Parser fixes needed for full test enablement.

lang/src/compiler/entry/using_resolver_box.hako

@@ -211,8 +211,8 @@ static box Stage1UsingResolverBox {
 
   _read_file(path) {
     if path == null { return null }
-    @fb = new FileBox()
+    local fb = new FileBox()
-    @ok = fb.open(path, "r")
+    local ok = fb.open(path, "r")
     if ok != 1 {
       // Dev note: keep quiet by default; enable verbose by setting HAKO_STAGEB_USING_DEBUG=1.
       local dbg = env.get("HAKO_STAGEB_USING_DEBUG")
@@ -221,7 +221,7 @@ static box Stage1UsingResolverBox {
       }
       return null
     }
-    @content = fb.read()
+    local content = fb.read()
     fb.close()
     return content
   }