Breaking release: match locations are reported by integer sequence id (SeqId)
instead of by FASTA header string, removing one String allocation per located
occurrence. Header <-> id lookup is O(1) in both directions via seq_header /
seq_id, and sequence headers must now be unique (FmIndexError::DuplicateHeader).

The on-disk index format is unchanged, but an index built by 0.1.0 from
duplicate headers is now rejected on load and must be rebuilt.

See CHANGELOG.md for the full entry.