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.

include/screencast/render/error.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <variant>
+
+namespace sc {
+
+struct RendererError {
+    std::string message;
+};
+
+// C++20 does not provide std::expected. Use a variant-based result type so
+// fallible renderer operations do not rely on exceptions.
+template <typename T> using RendererResult = std::variant<T, RendererError>;
+
+template <typename T> constexpr bool is_renderer_error(const RendererResult<T>& result) noexcept {
+    return std::holds_alternative<RendererError>(result);
+}
+
+template <typename T> T& renderer_value(RendererResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> const T& renderer_value(const RendererResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> RendererError& renderer_error(RendererResult<T>& result) {
+    return std::get<RendererError>(result);
+}
+
+template <typename T> const RendererError& renderer_error(const RendererResult<T>& result) {
+    return std::get<RendererError>(result);
+}
+
+} // namespace sc