#1429 made the first terminal status win, but the guard only refused
terminal to terminal changes. A worker's poll that reports DOWNLOADING
after a cancel still overwrote CANCELLED (the poll loop calls the status
callback before it waits on the cancel flag), and the COMPLETE or ERROR
that followed was then accepted as a first terminal status, so the
terminal hook ran a second time.
This refuses any status change away from a terminal status. `add()` and
`enqueue_existing()` reset the status themselves, so retries are
unaffected.
The new test cancels a downloading task, sends DOWNLOADING and then
COMPLETE, and expects CANCELLED and one hook call. It fails on main and
passes here. Full non-e2e suite passes, ruff and basedpyright are clean.
Found in a Claude pass over 1.5.0, same as my other small fixes from it.
splitsec2
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