fix: MIR builder me resolution for static box methods

- Fixed me ValueId inconsistency in static box methods
- Previously, each me reference generated a new const __me__ ValueId
- Now caches the first me ValueId in variable_map for reuse
- This ensures RefSet and RefGet operate on the same object
- ArrayBox get/set/push now working correctly in VM mode
- Test results: 1, 42, 3 (as expected)

🔧 Technical Details:
- build_me_expression() now stores fallback ValueId in variable_map
- Subsequent me references reuse the same ValueId
- VM BoxCall debug logs confirm ArrayBox methods dispatch correctly

Co-Authored-By: ChatGPT5
Co-Authored-By: Claude <noreply@anthropic.com>

src/boxes/array/mod.rs

@@ -302,6 +302,7 @@ impl Display for ArrayBox {
 }
 
 impl NyashBox for ArrayBox {
+    fn is_identity(&self) -> bool { true }
     fn clone_box(&self) -> Box<dyn NyashBox> {
         Box::new(self.clone())
     }