Add ZeroGPU stub so Space boots

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app.py

@@ -1,4 +1,12 @@
 import gradio as gr
+import spaces
+
+
+# ponytail: never called; ZeroGPU hardware refuses to boot without one @spaces.GPU function. Delete if the Space moves
+# to cpu-basic (needs PRO to downgrade).
+@spaces.GPU
+def _zerogpu_stub():
+    pass
 
 SYSTEM_PROMPT = (
     "Use only the Three.js core ES module from\n"