Completes the JACK output on top of the foundation (fbc03f41):
- StreamType::Jack + the player dispatch (backend==Jack -> JackStream).
- PlaybackEngine::Jack: a feeder thread (mirrors AlsaDirect) that resamples each
source to the JACK graph rate (UniformSourceIterator, stereo) and writes it to
a lock-free f32 SPSC ring (ringbuf); wired at the 4 engine-creation sites.
- jack_backend.rs: f32-native ring (fixes a byte-alignment white-noise bug) plus
auto-connect of qbz:out_FL/FR to the physical playback on activation, so audio
reaches the hardware without a patchbay (a user may re-patch freely).
- Re-enabled the available_backends push (JACK is now offered in the selector).
Verified on hardware: select JACK -> QBZ registers as client 'qbz' with stable
ports qbz:out_FL/FR (client.api=jack) -> music plays through the DAC, resampled
to the graph rate (NOT bit-perfect; BitPerfectMode::Disabled, as designed). The
ALSA-exclusive / DAC-passthrough bit-perfect paths are untouched.
Slice 3.2 (pending, separate): JACK<->passthrough/exclusive mutual-exclusion UI,
the bit-perfect-OFF note, runtime graph-rate-change callback, mono/multichannel.
cc
vicrodh
68b6cfbcfa