[move-model] Document and optimize struct usage fallback path

Add documentation explaining that get_using_functions() now uses pre-computed data for source-built structs, and the expensive lazy fallback should rarely execute.

Initialize ghost memory structs with empty using_funs set to completely eliminate the fallback path execution.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: junxzm1990

third_party/move/move-model/src/model.rs

@@ -1989,7 +1989,8 @@ impl GlobalEnv {
             visibility: Visibility::Private,
             has_package_visibility: false,
             is_empty_struct: false,
-            using_funs: RefCell::new(None),
+            // Ghost memory structs are synthetic and not directly used by function bodies
+            using_funs: RefCell::new(Some(BTreeSet::new())),
         }
     }
 
@@ -4234,10 +4235,15 @@ impl<'env> StructEnv<'env> {
     }
 
     /// Returns the functions that use this struct.
+    ///
+    /// For structs built from source code, this information is pre-computed during the build
+    /// phase by tracking struct usage in function bodies (see module_builder.rs def_ana_fun).
+    /// Falls back to expensive lazy computation for other cases.
     pub fn get_using_functions(&self) -> BTreeSet<QualifiedId<FunId>> {
         if let Some(using) = &*self.data.using_funs.borrow() {
             return using.clone();
         }
+        // Fallback: compute lazily for structs not built from source
         let mut set = BTreeSet::new();
         let my_id = self.get_qualified_id();
         for module_env in self.module_env.env.get_modules() {