Fix: Align cache path with preload_from_hub behavior

Root cause (Git history analysis):
- preload_from_hub ALWAYS downloads to ~/.cache/huggingface/hub/ (hardcoded by HF Spaces)
- config.py was using HF_HOME env var → ~/.cache/huggingface (missing /hub suffix)
- Result: transformers looked in ~/.cache/huggingface/models--* but files were in ~/.cache/huggingface/hub/models--*

Solution:
- config.py now uses HF_HUB_CACHE environment variable
- HF_HUB_CACHE is set in README.md to /home/user/.cache/huggingface/hub
- This matches exactly where preload_from_hub downloads models
- Simplified logic: removed complex fallback, direct path alignment

Official documentation:
"Files are saved in the default huggingface_hub disk cache ~/.cache/huggingface/hub.
If your application expects them elsewhere or you changed your HF_HOME variable,
this pre-loading does not follow that at this time."
Source: https://huggingface.co/docs/hub/spaces-config-reference

Changes:
- code/cube3d/config.py lines 61-83: Use HF_HUB_CACHE instead of HF_HOME
- Removed complex write-test fallback logic
- Aligned with HF Spaces official behavior

Expected result:
✅ [Config] HuggingFace cache directory: /home/user/.cache/huggingface/hub
✅ CLIP model loaded successfully from preloaded cache

code/cube3d/config.py

@@ -58,42 +58,17 @@ DATA_DIR = PROJECT_ROOT / "data"
 CAR_1K_DIR = DATA_DIR / "car_1k"
 CAR_DATA_DIR = DATA_DIR / "1313个筛选车结构和对照渲染图"
 
-# HuggingFace model cache directory with robust fallback handling
-# Priority: /data/.huggingface (persistent storage) → /home/user/.cache/huggingface (backup)
+# HuggingFace model cache directory
+# CRITICAL: Must match where preload_from_hub downloads models
 if ENVIRONMENT == "huggingface":
-    # HuggingFace Spaces: Try persistent storage with graceful fallback
-
-    # Candidate directories in priority order
-    candidates = [
-        os.getenv("HF_HOME", "/data/.huggingface"),  # Persistent storage (paid tier)
-        "/home/user/.cache/huggingface",              # Ephemeral but always writable
-    ]
-
-    HF_CACHE_DIR = None
-    for cache_dir in candidates:
-        try:
-            # Attempt to create directory to verify write permissions
-            os.makedirs(cache_dir, exist_ok=True)
-
-            # Test write access by creating a test file
-            test_file = os.path.join(cache_dir, ".write_test")
-            with open(test_file, 'w') as f:
-                f.write("test")
-            os.remove(test_file)
-
-            # Success! Use this directory
-            HF_CACHE_DIR = cache_dir
-            print(f"✅ [Config] HuggingFace cache directory: {HF_CACHE_DIR}")
-            break
-
-        except (PermissionError, OSError) as e:
-            print(f"⚠️  [Config] Cannot use {cache_dir}: {e}")
-            continue
-
-    # Fallback if all candidates failed (should never happen)
-    if HF_CACHE_DIR is None:
-        HF_CACHE_DIR = tempfile.gettempdir()
-        print(f"⚠️  [Config] Using system temp directory as last resort: {HF_CACHE_DIR}")
+    # HuggingFace Spaces: Use HF_HUB_CACHE (matches preload_from_hub behavior)
+    # preload_from_hub ALWAYS downloads to ~/.cache/huggingface/hub regardless of HF_HOME
+    # See: https://huggingface.co/docs/hub/spaces-config-reference
+    HF_CACHE_DIR = os.getenv(
+        "HF_HUB_CACHE",
+        os.path.expanduser("~/.cache/huggingface/hub")
+    )
+    print(f"✅ [Config] HuggingFace cache directory: {HF_CACHE_DIR}")
 
 else:
     # Local development: use standard user cache