fix(hub): make the streamed download path reliable, and clean up after an abort

On Firefox and Safari the service worker is the only unbounded way to write a
download to disk: neither has the File System Access API, and OPFS is not a
substitute — measured on Firefox 154, its quota is exactly 10% of the volume's
size (389,233,459 bytes of a 3,892,334,592-byte volume, refused to the byte),
which a film exceeds. So when this path declines, a large download has nowhere
left to go, which makes its reliability a correctness property.

Four ways it declined, all of them avoidable:

- it was registered inside the first click on Download, so that click paid
  install, activate and claim while somebody watched a button do nothing;
- `_swReady` cached a null for the life of the page. One slow first click left
  the tab unable to stream anything again, curable only by a reload nobody knew
  to do. Only a successful controller is remembered now;
- control was waited for with a 3 s cap. It is 15 s, and a page that is active
  but not controlled asks the worker to claim again (`mbdl-claim`) instead of
  declaring the path unavailable;
- a missed navigation gave up at once. It gets a second attempt with a fresh id
  and iframe, the failed one torn down completely first.

Also closes a MessagePort leaked per download, and gives the reason a name
(`lastStreamFailure`) so a refusal can say what happened. The timeouts became
parameters: the defaults are the production values, no caller passes any, and
the tests do not spend a minute waiting.

`openTarget` gets an unrelated but adjacent fix, in the same file: it creates
the destination with `getFileHandle({create: true})`, so an empty file exists
before the first byte, and `abort()` leaves the target untouched — every
cancelled download left a 0-byte file behind, and since `freeName` avoids
collisions, three cancels left film.mkv, film (2).mkv and film (3).mkv, all
empty. Its `abort()` now removes the entry. Safe here and only here, because
`freeName` guarantees the name was not taken: the `showSaveFilePicker` path
must not do the same, where the person may have picked an existing file whose
contents `abort()` correctly preserves. Verified by hand in Chrome.

test_streamed_download_reliability.py runs the real module under Node against a
stubbed browser — it fails if the null is cached again.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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