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/Cargo.toml

@@ -18,7 +18,11 @@ tracing-subscriber = { workspace = true }
 async-trait = { workspace = true }
 bytes = { workspace = true }
 anyhow = "1"
-cpal = "0.15"
+cpal = { version = "0.15", optional = true }
+
+[features]
+default = []
+audio = ["cpal"]
 
 [[bin]]
 name = "wzp-client"