feat(app): add PLI feedback, jitter reordering, and hardware decode

Loss recovery for the streaming path:

- PLI over signaling: the depacketizer now reports damaged frames
  (DepacketizeResult) and the receiver asks the sender for a keyframe
  (SessionPli, rate-limited to one per 500 ms). The sender keeps the
  signaling channel open during the session and honors PLIs through
  the new thread-safe SenderPipeline::request_keyframe(). Recovery
  takes one frame time instead of waiting out the GOP.
- RtpJitterBuffer: reorders RTP packets by sequence number (16 packets
  / 60 ms) before the in-order depacketizer, so Wi-Fi reordering is
  not misread as loss; in-order streams release immediately, and a
  straggler older than the delivered sequence is discarded.
- Hardware H.264 decode probe: DecoderFactory tries h264_v4l2m2m (the
  VideoCore path on the Pi) with an automatic software fallback and a
  clear journal line for the chosen path; --swdecode opts out.

Validated: PLI end-to-end with a probe that drops a mid-keyframe
packet over real UDP (receiver logged the damaged frame and the PLI
arrived with the session id); hardware probe fails cleanly and falls
back on this desktop; jitter reordering covered by unit tests.
meson test 5/5 in both build configurations, valgrind clean.

include/screencast/app/cli.h

@@ -18,6 +18,7 @@ struct ReceiveCommand {
     int local_rtp_port = 5004;
     int signaling_port = 5005;
     bool fullscreen = false;
+    bool software_decode = false; // --swdecode disables the hardware probe
 };
 
 struct DiscoverCommand {

include/screencast/app/pipeline.h

@@ -39,6 +39,10 @@ class SenderPipeline {
     bool start();
     void stop();
 
+    // Ask the sender to encode its next frame as a keyframe. Thread-safe;
+    // used by the PLI feedback path.
+    void request_keyframe();
+
   private:
     class Impl;
     std::unique_ptr<Impl> impl_;