image_preprocessing: decode bitshuffle+LZ4 on the GPU

The pipeline decompressed each image on the host and uploaded the result. On
an 18 Mpx rotation dataset that made the host-to-device copy the bottleneck of
the whole per-image loop: nsys puts the copies at 78% of the loop against 39%
for every kernel combined - 3600 transfers of 72.4 MB - and they ran at only
12.5 GB/s of an available 27-28 because the host-side decompression was itself
saturating host memory bandwidth. The GPU was mostly waiting.

So the compressed chunk goes across instead, about 4 MB rather than 72 MB, and
is decoded on the device. That removes the transfer and the host decompression
that was throttling it, in one change. Measured on an idle machine, a run goes
from 45.11 s to 24.97 s - 1.81x - with the merged output unchanged.

THE APPROACH IS JON WRIGHT'S (ESRF): "Experiences with GPU decompression for
bitshuffle + LZ4 data", HDF5 User Group 2021, and github.com/jonwright/
bslz4decoders. The kernels here are ours, but the idea and the demonstration
that it is worth doing are his. Cited in docs/ACKNOWLEDGEMENT.md and in the new
section 0 of docs/CPU_DATA_ANALYSIS.md.

Two kernels mirror the CPU decoder. LZ4 runs one WARP per bitshuffle block:
every lane parses the same sequence stream (a broadcast read, no divergence)
and the literal and match copies are split across the 32 lanes so the stores
coalesce; an overlapping match is treated as a pattern of period offset sourced
from bytes that already precede the write position, which keeps it parallel
rather than a serial byte loop. One thread per block instead measured 13x
slower. The bitshuffle inverse then un-transposes each byte-plane through
shared memory and interleaves the planes back into elements.

Only BSHUF_LZ4 is decoded on the device. The zstd variants have no device
decoder, and neither has an uncompressed or float image; Supports() returns
false for those and the caller decompresses on the host exactly as before. The
fallback is explicit, so a format we cannot decode on the device is a slower
path and never a wrong answer.

Tests hold the device decoder against the CPU one byte for byte, on data from
the production compressor, for every element size the detectors emit -
including the 8-bit DECTRIS modes, which take bitshuf_decode_block's separate
elem_size == 1 branch - plus a many-block frame, the formats it must decline,
and malformed containers, which must throw rather than run off a buffer.

Battery: 37 crystals, no failures, identical to the host-decode run.

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

image_analysis/image_preprocessing/ImagePreprocessorGPU.h

@@ -3,7 +3,10 @@
 
 #pragma once
 
+#include <memory>
+
 #include "ImagePreprocessor.h"
+#include "BSLZ4DecoderGPU.h"
 #include "../indexing/CUDAMemHelpers.h"
 #include "../indexing/CudaSharedTables.h"
 
@@ -23,7 +26,12 @@ class ImagePreprocessorGPU : public ImagePreprocessor {
 
     std::vector<int32_t> cpu_image;
 
+    // Built on first use: a decoder that can serve this engine's images, sized to the frame.
+    std::unique_ptr<BSLZ4DecoderGPU> bslz4_decoder;
+
     template <class T> ImageStatistics Analyze(ImagePreprocessorBuffer &processed_image, const uint8_t *input, T err_value, T sat_value);
+    // Preprocess an image already sitting in gpu_decompressed_image, shared by both entry points.
+    template <class T> ImageStatistics AnalyzeOnDevice(ImagePreprocessorBuffer &processed_image, T err_value, T sat_value);
 public:
     // copy_image_to_host copies the preprocessed image back after every frame. It is only needed when
     // something on the CPU reads it - the GPU engines all work off the device buffer - and at 4 bytes
@@ -31,6 +39,8 @@ public:
     ImagePreprocessorGPU(const DiffractionExperiment &experiment, const PixelMask &mask, std::shared_ptr<CudaStream> stream,
                          bool copy_image_to_host = true);
     ImageStatistics Analyze(ImagePreprocessorBuffer &processed_image, const uint8_t *decompressed_image, CompressedImageMode image_mode) override;
+    bool AnalyzeCompressed(ImagePreprocessorBuffer &processed_image, const CompressedImage &image,
+                           ImageStatistics &stats) override;
     void PinInputBuffer(std::vector<uint8_t> &buffer, size_t size) override;
 };