feat(rdp): remote audio and a shared folder redirected as a drive

- Audio: the RDPSND channel with IronRDP's cpal playback backend (PCM;
  Opus left out to avoid another native library). On by default for new
  RDP profiles. Building now needs the ALSA headers on Linux
  (libasound2-dev), added to CI, release and fuzz jobs and the build docs.
- Shared folder (Linux/macOS): one local folder appears on the remote
  desktop as a drive. IronRDP's native backend joins server paths onto
  the share unchecked, so it is wrapped in ConfinedDrive: create, list
  and rename paths with '..' components, NUL bytes, or that resolve
  through a symlink outside the share are refused (ACCESS_DENIED);
  tests cover the escapes.

Verified against xrdp (with FUSE): the remote read the shared file, and
writes, mkdir and rename landed in the local folder. Not verified:
audio playback (the test container has no sound module), and xrdp lists
the drive as 'ignored' and can't list its directories (an IronRDP/xrdp
naming and listing mismatch, fixed upstream for the name).

rust/core/src/validation.rs

@@ -334,6 +334,10 @@ pub fn validate_rdp_config(cfg: &RdpConfig) -> Result<(), ValidationErrors> {
         }
         t => errs.add("rdp.proxyType", format!("unsupported proxy type {t:?}")),
     }
+    let share = cfg.shared_folder.trim();
+    if !share.is_empty() && !std::path::Path::new(share).is_dir() {
+        errs.add("rdp.sharedFolder", "the shared folder must be an existing folder");
+    }
     if cfg.ssh_tunnel {
         if !matches!(cfg.proxy_type.as_str(), "" | "none") {
             errs.add("rdp.proxyType", "use either a proxy or an SSH tunnel, not both");