feat(audio,linux): output via SDL2; cpal stays on Windows/macOS

User reported persistent crackling/popping from peer audio on Linux even
after fixing the 48k->device-rate resampler boundary discontinuities,
clamping pre-Opus-encode peaks, and pre-allocating the playback scratch
buffer. Logs confirmed cpal opened raw ALSA at 44.1k native, no callback
budget violations, no underrun warnings -- yet the audio was still poor.

Root cause: cpal on Linux opens raw ALSA's 'default' PCM. On modern
PipeWire / pipewire-alsa boxes that virtual device routes through ALSA's
dmix + plug layers, whose default resampler is nearest-neighbour. cpal
also picks a small default period size (~256 frames / 5.8 ms) leaving no
headroom for kernel scheduler jitter. Both effects compound into the
crackling the user heard.

Upstream tsclientlib's own audio example
(tsclientlib/examples/audio_utils/ts_to_audio.rs) and the official Qint
client both use SDL2 with AudioSpecDesired { freq: 48000, channels: 2,
samples: 960 }. SDL2 on the same systems routes through PipeWire's PA
bridge (or PulseAudio directly), both carrying high-quality resamplers.

Fix:
  * Add sdl2 = '0.37' as a target_os=linux dependency. Links libSDL2-2.0
    .so (Arch sdl2-compat over SDL3, Debian libsdl2-2.0-0, Fedora SDL2).
  * New module crates/chanora_audio/src/sdl_output.rs implementing
    SdlOutput: opens a 48 kHz stereo 960-frame callback that zeroes the
    buffer and calls AudioHandler::fill_buffer directly (no user-side
    resampler). Master gain + hard-mute atomics wired in identically to
    the cpal callback so set_output_gain / set_output_muted keep working.
  * engine.rs cfg-gated: target_os='linux' builds SdlOutput; everywhere
    else continues with the cpal output path (including the device-native-
    rate negotiation and resampler-continuity fixes shipped earlier --
    those remain correct on Windows/macOS where cpal targets WASAPI /
    CoreAudio cleanly).
  * The cpal output helpers (build_output_stream, PlaybackResampleState,
    FromF32) are now cfg(not(target_os='linux'))-gated so the Linux
    build doesn't emit dead-code warnings.

Capture path still cpal on every platform -- outbound audio was not
reported as bad. Resampler-continuity fix on the capture side stays:
microphone -> Opus encoder still goes through the linear interpolator
with the last-sample anchor.

Tests: 32 / 0 / 0 (chanora_audio), workspace 78 / 0 / 1 unchanged.

crates/chanora_audio/Cargo.toml

@@ -75,3 +75,21 @@ futures-util = { version = "0.3", default-features = false, features = ["std"] }
 # options. The portal recommends fresh tokens to scope its own
 # object paths per call.
 rand = "0.8"
+# SDL2 audio for Linux. Replaces the cpal capture / playback paths
+# on Linux only; cpal stays in use on Windows/macOS. Rationale: the
+# cpal Linux backend opens raw ALSA `default`, which on most Arch
+# / Fedora / Debian installs routes through `dmix` + `plug` with
+# nearest-neighbour resampling and very small period sizes — the
+# combination produces audible crackling/popping. SDL2 on the same
+# systems routes through PipeWire's PulseAudio compat bridge (or
+# real PulseAudio), both of which carry a high-quality resampler
+# and a sensible default period. The upstream tsclientlib audio
+# example (`tsclientlib/examples/audio_utils/ts_to_audio.rs`) and
+# the official Qint client both use SDL2 in exactly this shape;
+# this dep brings Chanora in line with that pattern.
+#
+# `bundled` is OFF deliberately — we link against the system
+# libSDL2.so. Arch ships `sdl2-compat`; Debian/Ubuntu ship
+# `libsdl2-2.0-0`; Fedora ships `SDL2`. The chanora-flutter Linux
+# build documentation lists this as a runtime dependency.
+sdl2 = { version = "0.37", default-features = false }