fix(core): drop video packets on a full UDP send buffer, don't fail the session

UdpTransport sockets are non-blocking, so a momentarily-full kernel send buffer
makes socket.send return WouldBlock (EAGAIN). submit_frame propagated that as a
fatal error, tearing the whole punktfunk/1 session down — observed when attaching
to an already-running source (a headless Steam session) that emits frames at full
rate the instant capture connects: the first burst saturates the tx queue and the
session dies before a single frame reaches the client.

The data plane is lossy + Leopard-FEC-protected and runs infinite-GOP with RFI
keyframes, so the real-time-correct response to a full tx queue is to DROP the
packet (the next frame / FEC recovers) — exactly what the recv path already does
for WouldBlock. Blocking would queue stale frames and add latency. Loopback/M1
paths are unaffected (LoopbackTransport never blocks; M1 tests stay green).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-core/src/transport/udp.rs

@@ -31,8 +31,17 @@ impl UdpTransport {
 
 impl Transport for UdpTransport {
     fn send(&self, packet: &[u8]) -> std::io::Result<()> {
-        self.socket.send(packet)?;
-        Ok(())
+        match self.socket.send(packet) {
+            Ok(_) => Ok(()),
+            // The kernel UDP send buffer is momentarily full (a frame burst saturated the
+            // tx queue — common right after attaching to an already-running source that
+            // emits at full rate). Drop this packet rather than fail the whole stream: the
+            // data plane is lossy + FEC-protected and the next frame/RFI keyframe recovers,
+            // whereas blocking would queue stale frames and add latency, and erroring tears
+            // the session down. Mirrors the `recv` WouldBlock handling above.
+            Err(e) if e.kind() == std::io::ErrorKind::WouldBlock => Ok(()),
+            Err(e) => Err(e),
+        }
     }
 
     fn recv(&self) -> std::io::Result<Option<Vec<u8>>> {