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/network/test_signaling.cpp

@@ -93,6 +93,27 @@ int main() {
     check(answer.session_id == "test-session-0001", "session id echoes");
     check(answer.rtp_endpoint.port == advertised_rtp_port, "rtp port in answer");
 
+    // PLI: the receiver side asks for a keyframe mid-session; the client
+    // must receive it with the session id intact.
+    std::promise<sc::SessionPli> pli_promise;
+    auto pli_future = pli_promise.get_future();
+    std::atomic<bool> pli_seen{false};
+    client->on_message([&](const sc::SignalingMessage& message) {
+        if (const sc::SessionPli* pli = std::get_if<sc::SessionPli>(&message)) {
+            if (!pli_seen.exchange(true)) {
+                pli_promise.set_value(*pli);
+            }
+        }
+    });
+
+    sc::SessionPli pli;
+    pli.session_id = "test-session-0001";
+    server->send(pli);
+
+    check(pli_future.wait_for(std::chrono::seconds(5)) == std::future_status::ready, "pli received");
+    const sc::SessionPli received_pli = pli_future.get();
+    check(received_pli.session_id == "test-session-0001", "pli session id echoes");
+
     client->disconnect();
     server->disconnect();