feat(f722): add usb mavlink transport

firmware/libs/kaptan-tx/src/lib.rs

@@ -1,7 +1,7 @@
 //! Bounded protocol-neutral transmit admission and ownership lifecycle.
 //!
 //! Protocol encoders own frame bytes. This crate owns only bounded priority
-//! admission and the transition from pending frame to active DMA ownership.
+//! admission and the transition from pending frame to active transport ownership.
 
 #![no_std]
 
@@ -9,7 +9,7 @@
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
 pub enum FrameClass {
     /// Heartbeat and current safety/health status. A pending vital frame may
-    /// displace any pending non-vital frame; active DMA storage is untouched.
+    /// displace any pending non-vital frame; active storage is untouched.
     Vital,
     /// Fault transitions and authoritative replies that best-effort samples
     /// cannot displace.
@@ -147,12 +147,12 @@ impl<Frame: Copy, const N: usize> TransmitQueue<Frame, N> {
         Ok(StartOutcome::Started)
     }
 
-    /// Releases the active frame after a terminal DMA completion.
+    /// Releases the active frame after the transport no longer borrows it.
     pub fn complete(&mut self) -> Option<PendingFrame<Frame>> {
         self.active.take()
     }
 
-    /// Releases the active frame after a terminal DMA fault.
+    /// Releases the active frame after a terminal transport fault.
     pub fn fail(&mut self) -> Option<PendingFrame<Frame>> {
         self.active.take()
     }
@@ -162,6 +162,14 @@ impl<Frame: Copy, const N: usize> TransmitQueue<Frame, N> {
         self.active.is_some()
     }
 
+    /// Borrows the frame currently owned by the active transport operation.
+    ///
+    /// Packetized transports use this to retain one selected frame while
+    /// advancing it across multiple bounded endpoint writes.
+    pub const fn active(&self) -> Option<&PendingFrame<Frame>> {
+        self.active.as_ref()
+    }
+
     fn take_next(&mut self) -> Option<PendingFrame<Frame>> {
         let vital = self
             .pending
@@ -203,6 +211,7 @@ mod tests {
             Ok(StartOutcome::Started)
         );
         assert!(queue.is_active());
+        assert_eq!(queue.active().map(|pending| pending.frame), Some(1));
 
         assert_eq!(
             queue.enqueue(PendingFrame {
@@ -212,6 +221,7 @@ mod tests {
             EnqueueResult::Enqueued
         );
         assert_eq!(queue.complete().unwrap().frame, 1);
+        assert!(queue.active().is_none());
     }
 
     #[test]