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.

Cargo.lock

@@ -384,6 +384,7 @@ dependencies = [
  "jni 0.21.1",
  "ndk-context",
  "rand 0.8.6",
+ "sdl2",
  "thiserror 2.0.18",
  "tokio",
  "tracing",
@@ -2884,6 +2885,29 @@ version = "1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
 
+[[package]]
+name = "sdl2"
+version = "0.37.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b498da7d14d1ad6c839729bd4ad6fc11d90a57583605f3b4df2cd709a9cd380"
+dependencies = [
+ "bitflags 1.3.2",
+ "lazy_static",
+ "libc",
+ "sdl2-sys",
+]
+
+[[package]]
+name = "sdl2-sys"
+version = "0.37.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "951deab27af08ed9c6068b7b0d05a93c91f0a8eb16b6b816a5e73452a43521d3"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "version-compare",
+]
+
 [[package]]
 name = "sec1"
 version = "0.7.3"
@@ -3796,6 +3820,12 @@ version = "0.2.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "accd4ea62f7bb7a82fe23066fb0957d48ef677f6eeb8215f372f52e48bb32426"
 
+[[package]]
+name = "version-compare"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "579a42fc0b8e0c63b76519a339be31bed574929511fa53c1a3acae26eb258f29"
+
 [[package]]
 name = "version_check"
 version = "0.9.5"