feat(audio): JACK backend foundation (#263 Tier 3 — player wiring pending)

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

crates/qbz-slint/src/settings.rs

@@ -235,7 +235,8 @@ fn device_is_bit_perfect(backend: AudioBackendType, device: &qbz_audio::AudioDev
             alsa_section(&device.id, device.is_default, label) == AlsaSection::BitPerfect
         }
         AudioBackendType::PipeWire => device.is_hardware,
-        AudioBackendType::Pulse | AudioBackendType::SystemDefault => false,
+        // JACK never bit-perfect (resampled to the graph rate); no per-device concept.
+        AudioBackendType::Pulse | AudioBackendType::SystemDefault | AudioBackendType::Jack => false,
     }
 }
 
@@ -245,6 +246,7 @@ fn backend_label(t: AudioBackendType) -> String {
         AudioBackendType::Alsa => "ALSA",
         AudioBackendType::Pulse => "PulseAudio",
         AudioBackendType::SystemDefault => "System default",
+        AudioBackendType::Jack => "JACK",
     }
     .to_string()
 }