[Feature] First-class PulseAudio/PipeWire audio backend

[lqdev]

Summary

Add first-class PulseAudio and PipeWire support as an optional audio backend, beyond the default ALSA compatibility layer that rodio uses.

Related: #131 (Meta-Epic: Cross-Platform Audio Playback)

Problem / Background

The MVP uses rodio's default ALSA backend on Linux. While PipeWire and PulseAudio provide ALSA compatibility layers that usually work, some configurations may have issues:

• Containers and WSL2 environments
• Custom PipeWire configurations that don't expose ALSA compat
• Audio routing through PulseAudio-only sinks

Users hitting these edge cases currently fall back to the external player backend, which loses fine-grained control.

Proposed Solution

1. Cargo Feature Flags

[features]
default = []
pulseaudio = ["cpal/jack"]  # or appropriate cpal feature

2. Backend Auto-Detection

fn detect_linux_audio_backend() -> AudioBackendType {
    // Check for PipeWire first (modern default)
    if std::process::Command::new("pw-cli").arg("info").output().is_ok() {
        return AudioBackendType::PipeWire;
    }
    // Check for PulseAudio
    if std::process::Command::new("pactl").arg("info").output().is_ok() {
        return AudioBackendType::PulseAudio;
    }
    // Fall back to ALSA
    AudioBackendType::Alsa
}

3. Build Dependencies

# PulseAudio development libraries
sudo apt-get install libpulse-dev

# PipeWire development libraries (usually provides PulseAudio compat)
sudo apt-get install libpipewire-0.3-dev

Files to Modify

File	Change
Cargo.toml	Add optional pulseaudio feature with cpal backend flag
src/audio/mod.rs	Add backend detection logic
scripts/install-build-deps.sh	Add optional PulseAudio dev package
.github/workflows/release.yml	Test with PulseAudio in CI (optional)
GETTING_STARTED.md	Document PulseAudio build option

Acceptance Criteria

• cargo build --features pulseaudio builds with PulseAudio support
• Auto-detection chooses correct backend based on available audio server
• Default build (no features) still works with ALSA only
• CI tests both ALSA and PulseAudio builds
• Documentation explains when and how to enable PulseAudio support

Implementation Notes

• cpal's PulseAudio support may require the jack or specific host features — check cpal docs
• PipeWire typically provides PulseAudio API compat, so PulseAudio support often covers PipeWire too
• This is a build-time choice, not runtime — feature flags control which cpal backend is compiled in
• Consider adding a config.audio.backend field: "auto", "alsa", "pulseaudio"