Use the default tree for the first block when ibuf < 256 entries,
matching the original's bFlag logic (ULTRACMP.CPP:1105). For larger
blocks, generate custom Huffman trees from actual symbol frequencies.
Compression improvement on text data (textfile.txt, 1719 bytes):
Before (default-only): 1688 bytes compressed
After (custom trees): 1066 bytes compressed (37% smaller)
All tests pass including the bidirectional DOSBox-X round-trip.
Eremey Valetov
b042b4b48b