fix(spa): nothing on the worker path may wait for ever

Four downloads on Firefox sat at "preparing" indefinitely, with the target queue
already bypassed there, so each opening was hanging on its own. The node journal
showed `d=0/8(q0) u=0/8(q0)` — no transfer had been asked for yet.

`_claimController` had two waits with no deadline at all,
`navigator.serviceWorker.register()` and `navigator.serviceWorker.ready`, while
SW_CONTROL_BUDGET_MS bounded only the wait that comes after them. `_swPromise`
is shared, so one unsettled wait left every download on the page suspended on
the same promise for the life of the tab.

Measured on Firefox 154, against a local 127.0.0.1 site so no hub was involved:
a worker that installs gives register() in 8ms and ready in 0ms; a worker whose
install handler rejects gives register() in 7ms and a `ready` that never settles
— still pending past ten seconds. register() resolves as soon as the
registration object exists, carrying nothing but an *installing* worker; ready
is what waits for an active one.

Every wait is now inside one budget, with two carve-outs so that a deadline
never costs a capability. A `ready` that times out while registration.active is
set is not fatal: ready may be waiting on a newer worker that cannot install
while an older one serves perfectly well. And the mbdl-claim recovery keeps its
own budget outside the deadline, because giving up there would cost Firefox the
only unbounded way it has to write a download to disk.

A deadline alone would have been a better-explained failure rather than a fix: a
registration stuck with nothing but an installing worker does not heal, and
every later visit finds the same one. So when ready times out with no active
worker, the registration is discarded and asked for once more with a fresh
budget, and the page repairs itself instead of needing developer tools.

Four cases pinned, each checked against the unfixed source. Hub suite 830
passed.

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

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