fix(spa): wake the download worker before handing it a stream

Reported from Chrome: a download started while an upload was running took
thirty seconds to begin, every time. The console named it exactly —

    /_mbdl/mtty5btz-sbmgdegx  404 ()
    [MeshBay] the worker did not answer the download within 15s (attempt 1)

A 404 from the hub means the request reached the *network*: the worker looked,
found no entry for that id and let it through. So the worker was alive and
controlling the page, and the message handing it the stream had simply never
been processed.

`pending` lives in the worker's memory, and a worker with nothing to do is
terminated within tens of seconds. A WebRTC upload gives it no events at all,
so minutes of uploading leave it dead; the stream posted to it is lost,
silently, and the iframe then wakes it with nothing to find.

`mbdl-ping` already existed for this exact reason -- sent every ten seconds
*while* writing, because a streaming response does not count as activity.
Nothing sent one before *starting*. So a download now wakes the worker and waits
for the pong, and `sw.js` answers `mbdl-ready` once it has actually stored the
entry, which the page waits for before navigating: confirmed rather than
assumed. A worker that predates the ack sends nothing and the page navigates
anyway, which is what it did before.

This cause was measured and wrongly dismissed hours earlier, with an idle probe
that made the worker work between its own attempts -- it never actually slept.
A measurement that does not reproduce the conditions refutes nothing. The
harness now models a worker that is asleep: a ping wakes it, and anything else
posted while it sleeps is lost, which is what made the failure silent.

`test_backpressure_is_real` read the first `worker.postMessage` in the function
to check that the readable half is transferred rather than copied. The wake-up
put a ping in front of it, so it began inspecting a call that carries only a
port -- and kept passing. It now checks every post, each bounded by its own
call, since the keep-alive ping transfers nothing at all. Same shape as the
upload-seal contract this morning: a guard that reads "the first" stops
guarding the moment something is inserted before it.

Hub suite 851 passed. Both new cases checked against the unfixed source.

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

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