Compressor: scale block size to a per-algorithm byte target

Replace the fixed-element DefaultBlockSize with a byte target divided by
elem_size to get the block element count, so the per-block working set (and
thus cache behaviour) stays constant across pixel bit depths instead of halving
from 8- to 16- to 32-bit. The target is per-algorithm, following the measured
sweet spots on sparse data: LZ4 wants a small, cache-resident block for
throughput (16 kB), ZSTD/RLE want a large block for ratio (128 kB). The gap is
widest on extreme-sparsity inputs such as the uint32 pixel_mask, where
large-block ZSTD reaches 100-1800x vs ~160x for LZ4.

The block size is read back per-dataset from the bitshuffle stream header
(block_size = header_bytes / elem_size) and the HDF5 filter params, so the
decompressor and external readers (XDS/Neggia/Durin/CrystFEL) need no change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compression/MaxCompressedSize.cpp

@@ -9,10 +9,10 @@
 int64_t MaxCompressedSize(CompressionAlgorithm algorithm, int64_t pixels_number, uint16_t pixel_depth) {
     switch (algorithm) {
         case CompressionAlgorithm::BSHUF_LZ4:
-            return bshuf_compress_lz4_bound(pixels_number, pixel_depth, JFJochBitShuffleCompressor::DefaultBlockSize) + 12;
+            return bshuf_compress_lz4_bound(pixels_number, pixel_depth, JFJochBitShuffleCompressor::BlockSize(algorithm, pixel_depth)) + 12;
         case CompressionAlgorithm::BSHUF_ZSTD:
         case CompressionAlgorithm::BSHUF_ZSTD_RLE:
-            return bshuf_compress_zstd_bound(pixels_number, pixel_depth,JFJochBitShuffleCompressor::DefaultBlockSize) + 12;
+            return bshuf_compress_zstd_bound(pixels_number, pixel_depth, JFJochBitShuffleCompressor::BlockSize(algorithm, pixel_depth)) + 12;
         default:
             return pixels_number * pixel_depth;
     }