feat(audio): add guarded ALSA hardware volume control

Probe exact mixer element identities, retain stable device routes, and fail closed for ambiguous or switch-only controls. Synchronize player volume through ALSA events without changing direct PCM configuration.

cx

crates/qbz-audio/Cargo.toml

@@ -50,6 +50,7 @@ alsa = "0.11"
 # Kept in lockstep with alsa (0.11 -> alsa-sys 0.4); a single alsa-sys is
 # required because of the `links = "alsa"` constraint shared with cpal.
 alsa-sys = "0.4"
+libc = "0.2"
 # zbus for org.freedesktop.ReserveDevice1 D-Bus client (device reservation).
 # Pinned to v4 to share the transitive copy already pulled in by
 # secret-service/souvlaki and avoid duplicate zbus compiles.