Backend-side foundation for a native JACK output. The hard, RT-sensitive part
is done and compiles: jack_backend.rs holds a JackStream that opens a JACK client
'qbz' with stable ports qbz:out_FL/FR, a lock-free SPSC ring buffer, and an
allocation-free process callback (de-interleaves stereo f32 to the ports). The
feeder pushes interleaved f32 via write_f32().
Also: jack 0.13 dep (linux), AudioBackendType::Jack + its factory arm + the
qbz-slint label/bit-perfect match arms. NOT yet offered in the selector
(available_backends push commented) because the player wiring is still pending:
StreamType::Jack + the dispatch branch (backend==Jack -> JackStream) + a
PlaybackEngine::Jack feeder thread that resamples each track to the JACK graph
rate. JACK mode will be opt-in and NOT bit-perfect (one fixed graph rate); the
ALSA-exclusive / DAC-passthrough bit-perfect paths are untouched.
TODO(distribution): feature-gate jack for Flatpak/distro builds without libjack.
cc
vicrodh
fbc03f41b7