feat(audio): route libasound errors through log::debug

libasound's default error handler prints directly to stderr. During
device enumeration probes, PCMs that wrap dmix or surround routes emit
benign errors ("unable to open slave", "no matching channel map") that
cluttered the app log without indicating a real problem.

Install a process-wide snd_lib_error_set_handler shim from
AudioBackendFactory::create_backend (idempotent via std::sync::Once)
that forwards the file/line/function/err/fmt tuple to log::debug. The
messages stay available with RUST_LOG=debug and disappear during
normal runs.

Adds alsa-sys 0.3 as a direct Linux-only dependency on qbz-audio
(previously transitive via alsa/cpal). The handler is declared
non-variadic and transmuted into libasound's variadic function pointer;
ignoring the unused varargs is ABI-compatible on every platform we
support.

cc

src-tauri/Cargo.lock

@@ -4791,6 +4791,7 @@ name = "qbz-audio"
 version = "0.1.0"
 dependencies = [
  "alsa",
+ "alsa-sys",
  "async-trait",
  "coreaudio-rs 0.14.1",
  "dirs",