Auto-launch ffplay on mirroring, fix reconnection port leak, fix audio seq wraparound, optimize CI

[Copilot]

Screen mirroring required manually launching ffplay, reconnection after stop failed (port 7000 stayed bound), audio stuttered every ~25 min at the 16-bit sequence number wraparound, and CI rebuilt FFmpeg/fdk-aac from scratch every run.

Auto-launch/kill ffplay on mirroring lifecycle

• Added OnMirroringStarted/OnMirroringStopped events through IRtspReceiver → AirPlayReceiver → AirPlayService
• VideoOutputService redesigned with StartMirroring()/StopMirroring() — creates pipe, launches ffplay, buffers frames until connection, kills ffplay on stop
• Pre-buffers SPS/PPS+IDR frames before ffplay connects, flushes from latest keyframe on connection
• Low-latency ffplay flags: -fflags nobuffer+discardcorrupt -flags low_delay -framedrop -avioflags direct -vf setpts=0

Fix reconnection failure (BaseTcpListener port leak)

StopAsync() only cancelled the token but never called _listener.Stop(), leaving port 7000 bound. Old dead listener accepted new connections which immediately closed → "0 frames written."

public override Task StopAsync()
{
    _cancellationTokenSource.Cancel();
    try { _listener.Stop(); } catch { }
    return Task.CompletedTask;
}

Fix audio sequence number wraparound

Three places in RaopBufferQueue used plain < on 16-bit sequence numbers, breaking at 65535→0:

// Before: seqnums 1,2,3… after wrap discarded as "old"
if (seqnum < FirstSeqNum && seqnum != 0) { ... }

// After: proper modular comparison
private static bool SeqBefore(ushort s1, ushort s2) => ((short)(s1 - s2)) < 0;

CI caching

actions/cache@v4 for FFmpeg binaries and built libfdk-aac DLL — skips ~100MB download and MSYS2 source build on cache hit.

README updated

Both EN/ZH READMEs updated: download link → GitHub Releases, simplified quick start (no manual ffplay), documented auto-launch behavior.

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