dendro v0.3.0

Changed

• Breaking: Frame is #[non_exhaustive]. A match on it needs a
  wildcard arm; adding a frame kind is then not a breaking change.
• FrameReader::next_frame skips a frame of a kind it does not know and counts
  it in the new FrameReader::skipped, as WIRE.md §7 already said it did. It
  returned an error, so a subscriber older than its publisher stopped at the
  first new kind rather than losing only what that kind carried.
  decode_payload on a single payload still refuses an unknown kind.

Fixed

• verify returned an error when SQLite's integrity check ran into the damage
  it was walking and failed partway with SQLITE_CORRUPT. That is the check
  finding corruption, and it is now reported as Problem::Corrupt like any
  other line. Damage to page 2, the first pointer-map page, reproduced it on
  every archive; a new test sweeps a damaged page across the file.