fix: make cpal/ALSA optional — headless Linux builds work without libasound

- cpal is now behind an 'audio' feature flag (off by default)
- --live mode requires --features audio at build time
- --send-tone and --record work on headless servers without audio libs
- Linux build script no longer installs libasound2-dev

Build for headless: cargo build --release
Build with mic/speakers: cargo build --release --features audio

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-client/src/cli.rs

@@ -134,7 +134,14 @@ async fn main() -> anyhow::Result<()> {
     let transport = Arc::new(wzp_transport::QuinnTransport::new(connection));
 
     if cli.live {
-        run_live(transport).await
+        #[cfg(feature = "audio")]
+        {
+            return run_live(transport).await;
+        }
+        #[cfg(not(feature = "audio"))]
+        {
+            anyhow::bail!("--live requires the 'audio' feature (build with: cargo build --features audio)");
+        }
     } else if cli.send_tone_secs.is_some() || cli.record_file.is_some() {
         run_file_mode(transport, cli.send_tone_secs, cli.record_file).await
     } else {
@@ -326,6 +333,7 @@ async fn run_file_mode(
 }
 
 /// Live mode: capture from mic, encode, send; receive, decode, play.
+#[cfg(feature = "audio")]
 async fn run_live(transport: Arc<wzp_transport::QuinnTransport>) -> anyhow::Result<()> {
     use wzp_client::audio_io::{AudioCapture, AudioPlayback};