dendro v0.2.1

Changed

• Frame::Rows.seq is a plain sequence number — strictly increasing,
  contiguous, one per frame, not a timestamp — and a gap means frames lost in
  transit. This reverses guidance added earlier in this section, which
  recommended an interval index; that recommendation was wrong and is removed.

  It contradicted rule 6, in the same sentence. Rule 6 already requires a frame
  per interval, empty when nothing was observed, so there is no skipped interval
  for a counter to miss — the empty frame is how "nothing was observed" is
  stated. An interval index merges that fact with "a frame did not arrive",
  losing the distinction, and aliases on sub-interval jitter: a reading taken
  slightly early lands in the previous bucket, which invents gaps that did not
  happen and hides gaps that did.

  What survives from that guidance is that seq need not start at zero,
  which the code never required.