fix(render): keep the whole SDL lifecycle on the render thread

The receiver window never appeared on Wayland. Two causes:

- The window was created on the main thread while event pumping and
  presenting ran on the render thread. SDL's Wayland backend requires a
  window's creation, event processing, drawing, and destruction to
  happen on one thread; a cross-thread surface simply never maps, with
  no error reported. The renderer now lives entirely on the render
  thread, with a condition-variable handshake so ReceiverPipeline::
  start() still reports renderer failures and timeouts.
- On Wayland a window is invisible until the first render commit, so
  even a healthy receiver showed nothing while waiting for a stream.
  The renderer presents one blank frame at init: the window is visible
  immediately, black until video arrives.

Also log 'stream started (WxH)' when the first frame decodes and the
first rendering failure, which is what made the remaining debugging
observable.

Validated headlessly end-to-end: a synthetic solid-color RTP feed drove
the real receiver over localhost UDP; the window mapped, reported
'stream started (320x240)', and a grim screenshot of the window region
showed the fed color (V=198, SATAVG=74 during the red feed). Phase 5
marked complete in docs/PHASES.md; current phase is now Phase 6.

docs/PHASES.md

@@ -54,10 +54,10 @@ H.264 bitstream.
 
 **Goal**: send RTP packets over UDP and render the result locally.
 
-- Implement `RtpTransport` with ASIO or raw UDP sockets.
-- Wire sender pipeline: capture → encode → RTP → localhost UDP.
-- Wire receiver pipeline: localhost UDP → RTP → decode → renderer.
-- Add SDL2/3 dependency and a minimal `Renderer`.
+- [x] Implement `RtpTransport` with ASIO or raw UDP sockets.
+- [x] Wire sender pipeline: capture → encode → RTP → localhost UDP.
+- [x] Wire receiver pipeline: localhost UDP → RTP → decode → renderer.
+- [x] Add SDL2/SDL3 dependency and a minimal `Renderer`.
 
 **Validation**: `screencast --send` and `screencast --receive` on the same
 machine show the captured desktop in a window.
@@ -89,4 +89,4 @@ where available.
 
 ## Current phase
 
-Phase 5 — Local UDP Sender → Receiver Loopback.
+Phase 6 — LAN Signaling and Discovery.