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.

tests/app/test_loopback.cpp

@@ -74,13 +74,13 @@ struct ReceiverSink {
     int last_height = 0;
 
     void on_packet(sc::RtpPacket packet) {
-        std::optional<std::vector<std::byte>> access_unit = depacketizer.depacketize(packet);
-        if (!access_unit.has_value()) {
+        const sc::DepacketizeResult result = depacketizer.depacketize(packet);
+        if (!result.access_unit.has_value()) {
             return;
         }
 
         sc::EncodedFrame encoded;
-        encoded.data = std::move(*access_unit);
+        encoded.data = std::move(*result.access_unit);
         encoded.rtp_timestamp = packet.header.timestamp;
 
         auto decoded_result = decoder->decode(encoded);