Fix: Restore demo.py to use correct config file (open_model_v0.5.yaml)

- Changed config_path from open_model.yaml back to open_model_v0.5.yaml
- open_model_v0.5.yaml is the validated production config (5-layer GPT)
- Fixes dimension mismatch error (ldr_proj 959 vs 2048)
- This matches the original demo configuration

code/demo.py

@@ -57,7 +57,7 @@ def get_gpt_engine_cached():
     print("🔧 Initializing GPT engine (one-time setup)...")
 
     # Use absolute path relative to this file (fixes HF Spaces deployment)
-    config_path = os.path.join(os.path.dirname(__file__), 'cube3d/configs/open_model.yaml')
+    config_path = os.path.join(os.path.dirname(__file__), 'cube3d/configs/open_model_v0.5.yaml')
     gpt_ckpt_path = None  # test mode doesn't use this
 
     # Detect HuggingFace Spaces environment