feat(app): implement Phase 5 local UDP sender->receiver loopback

Wire the first end-to-end pipeline: capture -> encode -> packetize ->
UDP -> depacketize -> decode -> render.

- UdpRtpTransport: raw POSIX UDP sockets (IPv4 via getaddrinfo), a
  receive jthread woken by socket close on stop; port 0 skips binding
  so the sender uses an OS-assigned source port. ASIO stays deferred
  to the signaling phase per ARCHITECTURE.md.
- SdlRenderer: SDL3 window/renderer with RGBA texture upload; the
  texture is recreated on resolution change. RendererFactory now
  returns RendererResult so SDL init failures carry a message,
  mirroring the codec/capture error patterns.
- screencast binary: parse_cli plus SenderPipeline/ReceiverPipeline
  per the app scaffolds; the sender creates its encoder once capture
  reports real dimensions, the receiver keeps a bounded 3-frame queue
  to hold latency down and renders on its own thread until the window
  closes. cli argv signature fixed to 'const char* const*' so main's
  argv converts implicitly.
- Encoder: drop AV_CODEC_FLAG_GLOBAL_HEADER so libx264 repeats SPS/PPS
  in-band at each keyframe -- the receiver decodes from the bitstream
  alone, which also makes mid-stream joins and later PLI recovery
  work without out-of-band parameter negotiation. The round-trip test
  now exercises exactly that path.
- tests: new udp-loopback integration test pushes synthetic frames
  through a real localhost socket and decodes 10/10 frames with the
  right dimensions; valgrind clean (loopback + codec). meson test 4/4.

Manual validation on the desktop (receiver window shows the captured
desktop) is documented in docs/RUNBOOK.md.

tools/capture_smoke.cpp

@@ -97,9 +97,8 @@ int main(int argc, char** argv) {
             }
             encoder = std::move(sc::codec_value(encoder_result));
 
-            // The encoder is configured with AV_CODEC_FLAG_GLOBAL_HEADER, so
-            // parameter sets live in extradata; prepend them so the output
-            // file is a self-contained Annex-B stream.
+            // The encoder repeats SPS/PPS in-band at each keyframe, so the
+            // file is a self-contained Annex-B stream without a prefix.
             const auto extradata = encoder->get_extradata();
             if (!extradata.empty()) {
                 if (!write_bytes(output, extradata)) {