[feat](trx-core): add audio protocol types and framing helpers

Add audio streaming wire protocol: message type constants (StreamInfo,
RxFrame, TxFrame), AudioStreamInfo struct, and async read/write helpers
for length-prefixed frames over AsyncRead/AsyncWrite.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-core/src/audio.rs

@@ -0,0 +1,54 @@
+// SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+//! Audio streaming protocol types and framing helpers.
+//!
+//! Wire format: `[1 byte type][4 bytes BE length N][N bytes payload]`
+
+use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
+
+pub const AUDIO_MSG_STREAM_INFO: u8 = 0x00;
+pub const AUDIO_MSG_RX_FRAME: u8 = 0x01;
+pub const AUDIO_MSG_TX_FRAME: u8 = 0x02;
+
+/// Maximum payload size (1 MB) to reject bogus frames early.
+const MAX_PAYLOAD_SIZE: u32 = 1_048_576;
+
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct AudioStreamInfo {
+    pub sample_rate: u32,
+    pub channels: u8,
+    pub frame_duration_ms: u16,
+}
+
+/// Write a length-prefixed audio message.
+pub async fn write_audio_msg<W: AsyncWrite + Unpin>(
+    writer: &mut W,
+    msg_type: u8,
+    payload: &[u8],
+) -> std::io::Result<()> {
+    let len = payload.len() as u32;
+    writer.write_u8(msg_type).await?;
+    writer.write_u32(len).await?;
+    writer.write_all(payload).await?;
+    writer.flush().await?;
+    Ok(())
+}
+
+/// Read one length-prefixed audio message, returning `(type, payload)`.
+pub async fn read_audio_msg<R: AsyncRead + Unpin>(
+    reader: &mut R,
+) -> std::io::Result<(u8, Vec<u8>)> {
+    let msg_type = reader.read_u8().await?;
+    let len = reader.read_u32().await?;
+    if len > MAX_PAYLOAD_SIZE {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            format!("audio frame too large: {} bytes", len),
+        ));
+    }
+    let mut payload = vec![0u8; len as usize];
+    reader.read_exact(&mut payload).await?;
+    Ok((msg_type, payload))
+}

src/trx-core/src/lib.rs

@@ -2,6 +2,7 @@
 //
 // SPDX-License-Identifier: BSD-2-Clause
 
+pub mod audio;
 pub mod client;
 pub mod math;
 pub mod radio;