v0.6.1

Fixed

• gapblock_matrix's out-of-alphabet error named only the bad symbol, not which sequence it
  came from ("symbol '_' is not in the alphabet") — unhelpful when a single malformed row (e.g.
  a junction_aa containing a legacy out-of-frame marker) is buried in a batch of hundreds of
  thousands. The message now names the side, index, and offending string:
  "queries[42] ('CASSIRS_YEQYF'): symbol '_' is not in the alphabet". No change to which symbols
  are accepted (still the standard 20 + B/Z/X/* for alphabet="aa").