Fix file transfer, signaling reliability, and DC callback races

- wireFileRecv: replace OnMessage/OnClose channel approach with pion
  Detach API so SCTP buffers data regardless of callback timing — fixes
  0-byte files caused by pion dispatching data goroutines before OnMessage
  was registered
- yaw DC: same Detach API treatment for the control channel
- connectTo: use DetachDataChannels SettingEngine, replace stale PCs
  (Connecting/Failed/Disconnected state) instead of returning early;
  isCurrent guard in OnConnectionStateChange prevents stale-PC close
  from firing OnDisconnected
- offerByOrder: gate maybeOffer on peer ID order so only one side
  creates the yaw DC and SDP offer — fixes signaling glare causing
  triple connected (verified) logs
- pendingRecvs: promote single pendingRecv slot to map[string]*recvState
  so concurrent file offers don't overwrite each other
- peer-left: stop calling resetPeer() so active file DCs aren't closed
  before data arrives
- signaling ping: wrap conn.Ping with 10s timeout context so a dead TCP
  connection is detected within 30s rather than hanging forever
- signaling read: 3-minute read deadline forces reconnect if anchor
  silently evicts the peer from the room (idle timeout)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cli/cmd/flit/daemon.go

@@ -115,6 +115,7 @@ func runDaemon(ctx context.Context) {
 // reconnecting with exponential backoff whenever the connection drops.
 func runPeerLoop(ctx context.Context, tc transport.Config, myID string, peer peerConfig, downloadDir string) {
 	room := transport.PairRoomName(myID, peer.ID)
+	log.Printf("[%s] room: %s", peer.Label, room)
 	backoff := 2 * time.Second
 
 	for {

cli/cmd/flit/main.go

@@ -1,5 +1,6 @@
 // flit — headless CLI for ephemeral, E2E-encrypted file transfer.
 //
+//	flit id                   print this device's peer ID (hex Ed25519 pubkey)
 //	flit send <path>          generate a one-shot room, print QR + invite, wait for a peer, send
 //	flit recv <invite|code>   join a room from an invite string, accept the offered file
 //	flit daemon               run as a persistent receiver for trusted peers (reads ~/.flit/daemon.toml)
@@ -100,6 +101,8 @@ func main() {
 	defer stop()
 
 	switch os.Args[1] {
+	case "id":
+		printID()
 	case "send":
 		if len(os.Args) < 3 {
 			fmt.Println("usage: flit send <path>")
@@ -115,11 +118,20 @@ func main() {
 	case "daemon":
 		runDaemon(ctx)
 	default:
-		fmt.Println("usage: flit send <path> | flit recv <invite> | flit daemon")
+		fmt.Println("usage: flit id | flit send <path> | flit recv <invite> | flit daemon")
 		os.Exit(1)
 	}
 }
 
+func printID() {
+	sess, err := transport.NewSession(dataDir(), transport.Config{})
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "flit:", err)
+		os.Exit(1)
+	}
+	fmt.Println(sess.PeerID())
+}
+
 func send(ctx context.Context, path string) {
 	if _, err := os.Stat(path); err != nil {
 		fmt.Fprintln(os.Stderr, "flit:", err)