CUDA: the engines' setup copies belong on the engine's stream

Making the worker streams non-blocking removed the implicit ordering that the
constructors were still relying on. Each engine uploads its static inputs - the
pixel mask, the pixel-to-bin map, the corrections, the ROI map - with a blocking
NULL-stream cudaMemcpy, and then reads them from kernels on its own stream. A
pageable host-to-device cudaMemcpy returns once the source has been staged, with
the DMA still in flight, and a non-blocking stream no longer waits for the NULL
stream. The failure mode is a silently unapplied mask or a stale mapping, not a
crash, so it would not have announced itself.

Put them on the stream the engine already owns, and synchronise once at the end of
the constructor - that is required for the preprocessor, whose source is a local
vector, and leaves the others settled rather than in flight for the cost of one
one-time sync. The GPU spot-finder test uploaded its image the same way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

image_analysis/image_preprocessing/ImagePreprocessorGPU.cu

@@ -103,7 +103,11 @@ ImagePreprocessorGPU::ImagePreprocessorGPU(const DiffractionExperiment &experime
     for (int i = 0; i < npixels; i++)
         mask_vec[i] = (mask.GetMask().at(i) != 0);
 
-    cudaMemcpy(gpu_mask, mask_vec.data(), npixels, cudaMemcpyHostToDevice);
+    // On this engine's stream, like every other operation it issues: the streams are non-blocking, so a
+    // NULL-stream copy is no longer ordered against the kernels that read the mask. Synchronise before
+    // leaving the constructor - mask_vec is a local and the copy must not outlive it.
+    cudaMemcpyAsync(gpu_mask, mask_vec.data(), npixels, cudaMemcpyHostToDevice, *stream);
+    cudaStreamSynchronize(*stream);
 
     // Setup GPU settings
     cudaDeviceProp prop{};