FSST: decompress straight into the builder's byte storage - #9097
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`append_to_builder` staged the decompressed heap in a temporary `ByteBufferMut` and then copied it into the builder. `FsstDecodePlan` hoists the parts of `fsst_decode_bytes` that describe the decode — the code stream, the per-row lengths and the total decoded size — so the decompression can target the builder's spare capacity directly via `append_decoded`, leaving one prefix sum over the lengths as the only work beyond the bulk `decompress_into`. `FSST_DECODE_SLACK` names the 7 spare bytes that keep `decompress_into` on its wide-store path through the final value; it is a performance knob, not a safety requirement. Signed-off-by: Robert Kruszewski <robert@spiraldb.com> Signed-off-by: Robert Kruszewski <github@robertk.io>
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FSST append_to_builder decodes directly into the builder storage without additional copy