marCCD frames are widened on the device

The marCCD reader read each frame's 16-bit scanlines into scratch and widened them to int32 on the
worker thread (marccd::ReadInto, ~20 % of the image-loop worker CPU on 4nwv), and the 4-byte image was
uploaded. ReadRawImage now reads the scanlines straight into the image's buffer and hands them over as
a new read-only CompressionAlgorithm::UINT16_WIDENED (mode Int32): two bytes a pixel cross PCIe, from
page-locked memory, and the preprocessing kernel widens them itself.

It is not CompressedImageMode::Uint16, which calls 65535 an error pixel; a marCCD's 65535 is its
saturation marker. So the kernel compares each pixel with int32's error marker and the int32 saturation
cap - the comparisons are made on the pixel's value, in int64, rather than in the storage type - and
decides every pixel exactly as for the int32 image the host would have widened it to. The host widening
(JFJochDecompressPtr) is the CPU path.

Test: ImagePreprocessorGPU_StoredEncodingsMatchCPU, with a frame carrying 65535 at a saturation of
65535, against the CPU preprocessor on the host-widened image.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3

image_analysis/MXAnalysisWithoutFPGA.cpp

@@ -121,9 +121,11 @@ void MXAnalysisWithoutFPGA::Analyze(DataMessage &output,
         // sees the same error it saw before any of this existed - but a GPU-side problem costs speed
         // rather than the acquisition. Unless the context is lost, which the host route cannot survive.
         cuda_throw_if_context_lost();
-        // The miniCBF encoding is decoded on the device whenever there is one; the host decoder is the
-        // CPU path, not a slower second try. A stream the device refuses, the host refuses as well.
-        if (output.image.GetCompressionAlgorithm() == CompressionAlgorithm::CBF_BYTE_OFFSET)
+        // The miniCBF and marCCD encodings are decoded on the device whenever there is one; the host
+        // decoder is the CPU path, not a slower second try. A stream the device refuses, the host
+        // refuses as well.
+        if (output.image.GetCompressionAlgorithm() == CompressionAlgorithm::CBF_BYTE_OFFSET
+            || output.image.GetCompressionAlgorithm() == CompressionAlgorithm::UINT16_WIDENED)
             throw;
         spdlog::warn("Device decoding failed ({}), falling back to host decompression", e.what());
         decoded_on_device = false;