Real bug report: user selects their RTL-SDR dongle in the WebUSB picker,
"nothing happens" — no error, no sweep, no visible change at all.
Root cause: WebUSB's transferIn/controlTransfer calls have no built-in
timeout. readSamples()'s bulk transferIn (called every sweep step, twice —
once to discard PLL-settle samples, once for real) had nothing bounding
it, so if the dongle doesn't actually stream data for any reason (this
init sequence has never been verified against real hardware), that
promise just never settles — indistinguishable from the page being frozen,
forever, with no way for the UI to ever surface an error.
Added withTimeout(), applied to: the bulk IQ read (4s — the one most
likely to actually hang during sweeping) and the whole open()/init
sequence as one unit (15s, since it's ~20 sequential unverified register
pokes). Also stopped silently falling back to default
interface/endpoint values when the device's USB descriptor doesn't expose
a bulk-IN endpoint as expected — now logs a warning so a real endpoint
mismatch is at least visible in DevTools instead of only surfacing as a
downstream hang.
4 new unit tests for withTimeout(). 306/306 frontend tests pass.
