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.

docs/RUNBOOK.md

@@ -49,6 +49,23 @@ PipeWire proxy operation ran without the thread-loop lock — all pw
 calls that send messages (connect, stream, core) must happen under
 `pw_thread_loop_lock`.
 
+## Sender / receiver loopback (Phase 5, manual)
+
+Two terminals on the same desktop session:
+
+```sh
+./build/src/app/screencast --receive        # terminal 1: window appears
+./build/src/app/screencast --send           # terminal 2: portal source picker
+```
+
+Expected: the receiver window shows the captured desktop in near real time.
+Stop either side with Ctrl-C. Optional flags: `--port`, `--peer HOST[:PORT]`,
+`--bitrate KBPS`, `--target window`.
+
+The headless equivalent runs as part of `meson test` (`udp loopback` test):
+synthetic frames → encode → packetize → localhost UDP → depacketize →
+decode, no portal or window involved.
+
 ## Formatting
 
 ```sh