Add DLPack debug logging to diagnose zero-copy

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

sexp_effects/primitive_libs/streaming_gpu.py

@@ -336,9 +336,16 @@ class GPUVideoSource:
                 # This keeps the frame on GPU without any CPU transfer
                 try:
                     gpu_frame = cp.from_dlpack(frame_tensor)
+                    # Log success once per source
+                    if not getattr(self, '_dlpack_logged', False):
+                        print(f"[GPUVideoSource] DLPack zero-copy SUCCESS - frames stay on GPU", file=sys.stderr)
+                        self._dlpack_logged = True
                     return GPUFrame(gpu_frame, on_gpu=True)
-                except Exception:
+                except Exception as dlpack_err:
                     # Fallback: convert via numpy (involves CPU copy)
+                    if not getattr(self, '_dlpack_fail_logged', False):
+                        print(f"[GPUVideoSource] DLPack FAILED ({dlpack_err}), using CPU copy fallback", file=sys.stderr)
+                        self._dlpack_fail_logged = True
                     frame_np = frame_tensor.asnumpy()
                     return GPUFrame(frame_np, on_gpu=True)
             else: