fix(spa): one save dialog per batch, not one per file

Selecting four files on Chrome produced a Save As dialog for the first, then
one for the second only after that file had finished, while the last two timed
out; on a later attempt the three remaining transfers appeared frozen.

Two things were going on. Opening the target inside `prepare` had removed the
accidental serialisation that `for (…) await downloadFile(e)` used to provide,
so `_openTargetInTurn` now queues the openings — but a queue whose head is an
unanswered dialog is a head-of-line block, which is what the "freeze" was.

The code already recovered from a picker with no gesture behind it by streaming
instead, on the `SecurityError` Chrome throws. That branch was never reached:
Chrome does not throw, it shows the dialog anyway and waits for a human. So
anything that has to wait its turn is now marked `batched`, and a batched
opening prefers the streamed path whatever the download mode says. The first
file of a batch — the one actually holding the gesture — still gets its dialog,
so the preference is honoured where it can be. For the rest there is no gesture
left to spend and nothing is lost by streaming: the file still lands on disk,
in the browser's own download folder. Only the choice of folder goes, and it
was not on offer. If the worker does not answer, a batched download falls back
to the dialog rather than failing.

Also logs which path led to a dialog. A dialog is the one outcome nobody can
diagnose after the fact — it looks the same whether it was asked for or fallen
back to — and the report this fixes needed three test cycles to narrow. The two
harnesses that lift `_openDownloadTarget` as text now route console.info to
stderr, since they parse stdout as JSON.

Measured against the deployed hub in Chrome 152: the streamed path serves the
hidden iframe in 2-3 ms on a normal load, after a hard reload (via the
`mbdl-claim` recovery already in `_claimController`), and twice in the same
document. Hub suite 824 passed.

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

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