sound: add StreamDirection enum and --streams CLI arg

Add a StreamDirection enum (Output, Input) with ValueEnum derive and a
Display impl, and a --streams field to SoundArgs that accepts
comma-separated stream direction values with output,input as the
default. Add 7 CLI parsing tests covering the common cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vhost-device-sound/src/args.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 //! An arguments type for the binary interface of this library.
 
+use std::fmt;
 use std::os::fd::RawFd;
 use std::path::PathBuf;
 
@@ -24,6 +25,24 @@ pub struct SoundArgs {
     #[clap(long)]
     #[clap(value_enum)]
     pub backend: BackendType,
+    /// Stream directions to enable (comma-separated).
+    #[clap(long, value_delimiter = ',', default_values_t = [StreamDirection::Output, StreamDirection::Input])]
+    pub streams: Vec<StreamDirection>,
+}
+
+#[derive(ValueEnum, Clone, Copy, Debug, Eq, PartialEq)]
+pub enum StreamDirection {
+    Output,
+    Input,
+}
+
+impl fmt::Display for StreamDirection {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.to_possible_value()
+            .expect("no values are skipped")
+            .get_name()
+            .fmt(f)
+    }
 }
 
 #[derive(ValueEnum, Clone, Copy, Default, Debug, Eq, PartialEq)]

vhost-device-sound/src/main.rs

@@ -34,6 +34,7 @@ mod tests {
     use clap::Parser;
     use rstest::*;
     use std::path::PathBuf;
+    use vhost_device_sound::args::StreamDirection;
     use vhost_device_sound::BackendType;
 
     use super::*;
@@ -56,6 +57,107 @@ mod tests {
         assert_eq!(args.socket_fd, Some(3));
     }
 
+    #[test]
+    fn test_cli_streams_output_only() {
+        let args: SoundArgs = Parser::parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "output",
+        ]);
+        assert_eq!(args.streams, vec![StreamDirection::Output]);
+    }
+
+    #[test]
+    fn test_cli_streams_input_only() {
+        let args: SoundArgs = Parser::parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "input",
+        ]);
+        assert_eq!(args.streams, vec![StreamDirection::Input]);
+    }
+
+    #[test]
+    fn test_cli_streams_both() {
+        let args: SoundArgs = Parser::parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "output,input",
+        ]);
+        assert!(args.streams.contains(&StreamDirection::Output));
+        assert!(args.streams.contains(&StreamDirection::Input));
+    }
+
+    #[test]
+    fn test_cli_streams_default() {
+        let args: SoundArgs = Parser::parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+        ]);
+        assert!(args.streams.contains(&StreamDirection::Output));
+        assert!(args.streams.contains(&StreamDirection::Input));
+        assert_eq!(args.streams.len(), 2);
+    }
+
+    #[test]
+    fn test_cli_streams_invalid() {
+        let result = SoundArgs::try_parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "foobar",
+        ]);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_cli_streams_duplicate() {
+        let args: SoundArgs = Parser::parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "output,output",
+        ]);
+        // Duplicates are accepted by clap; the contains() conversion in main.rs
+        // naturally deduplicates since it produces booleans.
+        assert!(args.streams.contains(&StreamDirection::Output));
+    }
+
+    #[test]
+    fn test_cli_streams_empty() {
+        let result = SoundArgs::try_parse_from([
+            "",
+            "--socket",
+            "/tmp/vhost-sound.socket",
+            "--backend",
+            "null",
+            "--streams",
+            "",
+        ]);
+        assert!(result.is_err());
+    }
+
     #[rstest]
     #[case::null_backend("null", BackendType::Null)]
     #[cfg_attr(