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_rtp.cpp

@@ -4,6 +4,7 @@
 
 #include <algorithm>
 #include <array>
+#include <chrono>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
@@ -288,13 +289,17 @@ void test_depacketize_roundtrip() {
 
     sc::H264Depacketizer depacketizer;
     std::optional<std::vector<std::byte>> completed;
+    bool any_dropped = false;
     for (const sc::RtpPacket& packet : packets) {
-        if (auto result = depacketizer.depacketize(packet)) {
+        const sc::DepacketizeResult result = depacketizer.depacketize(packet);
+        if (result.access_unit.has_value()) {
             check(!completed.has_value(), "only one completion");
-            completed = std::move(result);
+            completed = std::move(result.access_unit);
         }
+        any_dropped = any_dropped || result.frame_dropped;
     }
     check(completed.has_value(), "frame completed");
+    check(!any_dropped, "no dropped frames in a clean stream");
     check(equal_bytes(*completed, access_unit), "access unit round-trip");
 }
 
@@ -305,9 +310,11 @@ void test_depacketizer_drops_gapped_frames() {
     check(packets.size() == 3, "gap test packet count");
 
     sc::H264Depacketizer depacketizer;
-    check(!depacketizer.depacketize(packets[0]).has_value(), "first fu chunk accepted");
+    check(!depacketizer.depacketize(packets[0]).access_unit.has_value(), "first fu chunk accepted");
     // packets[1] is lost in transit; the tail cannot complete the frame.
-    check(!depacketizer.depacketize(packets[2]).has_value(), "tail after gap dropped");
+    const sc::DepacketizeResult tail = depacketizer.depacketize(packets[2]);
+    check(!tail.access_unit.has_value(), "tail after gap dropped");
+    check(tail.frame_dropped, "drop reported after gap");
 }
 
 void test_depacketizer_separate_frames() {
@@ -319,12 +326,79 @@ void test_depacketizer_separate_frames() {
     const std::vector<sc::RtpPacket> second = packetizer.packetize(make_frame(f2, 90000));
 
     sc::H264Depacketizer depacketizer;
-    const std::optional<std::vector<std::byte>> au1 = depacketizer.depacketize(first[0]);
-    check(au1.has_value() && equal_bytes(*au1, f1), "first frame");
+    const sc::DepacketizeResult au1 = depacketizer.depacketize(first[0]);
+    check(au1.access_unit.has_value() && equal_bytes(*au1.access_unit, f1), "first frame");
+    check(!au1.frame_dropped, "first frame not dropped");
 
     // Same RTP timestamp on purpose: the marker alone separates frames.
-    const std::optional<std::vector<std::byte>> au2 = depacketizer.depacketize(second[0]);
-    check(au2.has_value() && equal_bytes(*au2, f2), "second frame with same timestamp");
+    const sc::DepacketizeResult au2 = depacketizer.depacketize(second[0]);
+    check(au2.access_unit.has_value() && equal_bytes(*au2.access_unit, f2), "second frame with same timestamp");
+    check(!au2.frame_dropped, "second frame not dropped");
+}
+
+void test_jitter_buffer_in_order() {
+    sc::RtpJitterBuffer jitter;
+    sc::RtpPacket packet;
+    packet.header.sequence_number = 100;
+
+    std::vector<sc::RtpPacket> released = jitter.push(packet);
+    check(released.size() == 1 && released[0].header.sequence_number == 100, "in-order releases immediately");
+
+    packet.header.sequence_number = 101;
+    released = jitter.push(std::move(packet));
+    check(released.size() == 1 && released[0].header.sequence_number == 101, "next packet releases too");
+}
+
+void test_jitter_buffer_reorders() {
+    sc::RtpJitterBuffer jitter;
+
+    sc::RtpPacket first;
+    first.header.sequence_number = 1;
+    std::vector<sc::RtpPacket> released = jitter.push(std::move(first));
+    check(released.size() == 1 && released[0].header.sequence_number == 1, "first packet releases");
+
+    // Arrives ahead of its predecessor: held, not delivered.
+    sc::RtpPacket third;
+    third.header.sequence_number = 3;
+    released = jitter.push(std::move(third));
+    check(released.empty(), "gap holds packets");
+
+    sc::RtpPacket second;
+    second.header.sequence_number = 2;
+    released = jitter.push(std::move(second));
+    check(released.size() == 2, "held packets release in order");
+    check(released[0].header.sequence_number == 2 && released[1].header.sequence_number == 3,
+          "released in sequence order");
+}
+
+void test_jitter_buffer_overflow_and_stragglers() {
+    // Small depth: a persistent gap overflows the buffer and flushes what
+    // is there, so genuine loss reaches the depacketizer instead of
+    // stalling delivery.
+    sc::RtpJitterBuffer jitter(4, std::chrono::milliseconds{500});
+
+    sc::RtpPacket packet;
+    packet.header.sequence_number = 10;
+    check(jitter.push(std::move(packet)).size() == 1, "first releases");
+
+    std::vector<sc::RtpPacket> released;
+    for (std::uint16_t sequence = 12; sequence < 17; ++sequence) {
+        sc::RtpPacket missing;
+        missing.header.sequence_number = sequence;
+        for (sc::RtpPacket out : jitter.push(std::move(missing))) {
+            released.push_back(std::move(out));
+        }
+    }
+    check(released.size() == 5, "overflow flushes the backlog");
+    for (std::size_t i = 0; i < released.size(); ++i) {
+        check(released[i].header.sequence_number == 12 + i, "flushed in order");
+    }
+
+    // A straggler older than the delivered sequence is discarded, not
+    // re-inserted out of order.
+    sc::RtpPacket straggler;
+    straggler.header.sequence_number = 11;
+    check(jitter.push(std::move(straggler)).empty(), "straggler discarded");
 }
 
 void test_sequence_wrap() {
@@ -370,6 +444,9 @@ int main() {
     test_depacketize_roundtrip();
     test_depacketizer_drops_gapped_frames();
     test_depacketizer_separate_frames();
+    test_jitter_buffer_in_order();
+    test_jitter_buffer_reorders();
+    test_jitter_buffer_overflow_and_stragglers();
     test_sequence_wrap();
     test_default_config_randomizes();
     test_empty_inputs();