Class ValidationQueue

Run task after any previously-queued validation for datasetId resolves. task receives an AbortSignal that fires when ValidationQueue.cancel is called with THIS call's QueueEntry (typically from the owning instance's dispose()).

The entry is created synchronously and handed to the optional onStart callback BEFORE the first await, so the caller can capture its own handle and later cancel exactly that entry — even after a newer same-datasetId validation has replaced it as the map head. The returned promise still resolves only when the validation completes, so await serialize(...) keeps its "wait until done" contract.

If the abort fires while waiting in line, task is skipped entirely (the closure may have captured a now-disposed this).

Cancel the specific entry the caller owns by firing its AbortSignal — nothing more. If the entry is still queued, its chained then skips the task body via the abort.signal.aborted guard; if it is running, the task observes the signal and unwinds.

Identity-scoped on purpose: it acts ONLY on the caller's own entry, so an older store's dispose() can never cancel a newer same-datasetId store's validation.

Deliberately does NOT touch the map. Map cleanup is left entirely to serialize's finally head-guard (queues.get(datasetId) === entry). Deleting here would be wrong for a WAITING head: an aborted waiting entry still settles when its predecessor settles (keeping successors FIFO-chained via its promise), and removing it early would let the next serialize see previous === undefined and run concurrently with the still-live predecessor. Safe to call more than once and on an already-settled entry.