Hellishly Slow Level 13 DEFLATE Compression

DEFLATE level 13 is a deliberately impractical libdeflate compression level: the output remains standard DEFLATE, while the encoder spends far more time searching parse, Huffman, and block split choices. On Silesia it saves 86'990 bytes, 0.134%, over level 12 and runs 56.4x slower; this cost is acceptable only when data is compressed once and distributed many times.