Refuse option appends that were queued behind an implementation claim

The active-implementation check ran before the exclusive queue, so a claim taken
while the append waited let the live document mutate and then fail in publish,
leaving the room ahead of durable state. Re-check inside the queue, and add
tests for concurrent appends, the queued-claim window and commit-failure rollback.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>