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/lib.rs

@@ -6,11 +6,13 @@
 //!
 //! Targets: Android (JNI), Windows desktop, macOS/Linux (testing)
 
+#[cfg(feature = "audio")]
 pub mod audio_io;
 pub mod bench;
 pub mod call;
 pub mod handshake;
 
+#[cfg(feature = "audio")]
 pub use audio_io::{AudioCapture, AudioPlayback};
 pub use call::{CallConfig, CallDecoder, CallEncoder};
 pub use handshake::perform_handshake;