v0.6.9

Performance

• BidirFmIndex now runs the same SMEM/MEM sweeps as the run-length index: pruned MEM
  sweep, lockstep SMEM backward phase, a derived k-mer interval cache (at most 2^21
  intervals, built on construction and load, never serialized) and one batched locate per
  query.
• BidirFmIndex sweep steps extend only the query symbol's compatible codes instead of
  ranking every symbol, answer an extension inside one 64-row occ block from that block
  alone, and filter left-maximal MEM rows a block at a time. On 200 Mbp at -O3:
  find_mems 4.4x, find_mems with locate 3.7x, find_smems 1.3x faster than 0.6.8.

Fixed

• BidirFmIndex::from_bytes rejects forward and reverse halves that do not match.