fix(spa): the target queue must not be able to freeze a batch

Four downloads on Firefox all sat at "preparing", with the node journal showing
`d=0/8(q0) u=0/8(q0)` — not one transfer opened, so nothing had got past the
client's target opening.

Serialising those openings was new in d6c4808, and on Firefox it regressed what
had always worked: four openings that ran at the same time began waiting on the
slowest. `_targetQueue` is module-level and never reset, so an opening that
never settles leaves the page unable to start any download again until it is
reloaded.

Two bounds, both narrowings of the queue rather than of any capability:

Only an opening that could actually show a dialog joins it. Firefox and Safari
have no `showSaveFilePicker` at all, so nothing there can race anything and the
queue bought nothing while costing everything; they now bypass it entirely,
which restores the previous behaviour by construction rather than by tuning.

And no opening waits behind another for longer than TARGET_QUEUE_BUDGET_MS
(90s) — generous enough never to cut in front of a real dialog, finite because
the alternative is a download panel that only a reload can fix. Releasing early
is safe: whatever is ahead is still the only unbatched opening, so the released
one takes the streamed path and opens no second dialog.

Measured on Firefox 154 against the deployed hub before writing any of this:
`register` and `ready` return instantly, the page is controlled, and four
serialised openings are served in 5-18 ms. The streamed path was never the
delay; the queue was.

Both new cases were checked against the unfixed source: without the bypass the
peak concurrency is 1 instead of 4, and without the budget the stuck-opening
case hangs. Hub suite 826 passed.

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

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