Waveform block codecs: encode cost is the XOR/dictionary transform, not zstd vs LZ4; zstd-9 is the sweet spot

Benchmark: per-signal 16K-change blocks (varint time deltas + XOR-vs-previous or dictionary-indexed values, 2-state plane dropped). Each codec compresses both candidates and keeps the smaller; every row round-trips losslessly. 147 M changes, measured on three CPUs.

- Building the raw XOR/dict payload costs ~150-750 ns/change, depending on the CPU and the value width. The codec itself adds only ~7 ns/change for zstd-1/3, ~30 for zstd-9, and ~1.5 us for zstd-19. - Decode is dominated by rebuilding the values, so every codec decodes within ~10% of the others. - Size vs zstd-3: lz4 1.78x, lz4hc 1.17x, zstd-1 1.13x, zstd-9 0.83x, zstd-19 0.75x. - Conclusions: zstd-9 gives -17% size for +17% encode. zstd-19 only makes sense as a background recompress. LZ4 is no win for this payload, which is already delta-coded, mostly zeros and small. - A 2.1 GHz Xeon Silver ran ~1.6-2x slower than an EPYC 9R14 on identical blocks. Label the CPU when you quote per-change costs. - To use LZ4 on a box that has /usr/lib64/liblz4.so.1 but no headers, declare the four public functions yourself (LZ4_compressBound, LZ4_compress_default, LZ4_compress_HC, LZ4_decompress_safe) as extern "C" and link the .so.1 by path. No install needed.