feat(capture): implement Phase 3 PipeWire/portal desktop capture

Implement the xdg-desktop-portal ScreenCast backend via libportal: a
blocking portal handshake (interactive source picker), a PipeWire stream
on the portal's node enumerating BGRx/BGRA/RGBx/RGBA, and a latest-frame
slot handing frames to next_frame(). stop() is thread-safe; teardown
follows the order PipeWire requires. All proxy operations run under the
thread-loop lock to satisfy the protocol extension context checks
('impl_ext_end_proxy called from wrong context' otherwise).

The encoder now accepts padded strides for packed RGB inputs (real
PipeWire row pitches) and maps the new PixelFormat::Bgrx to
AV_PIX_FMT_BGRA.

Add tools/capture_smoke: a manual smoke tool (interactive, not in
meson test) that captures N frames, encodes them, and writes a
self-contained Annex-B elementary stream with prepended SPS/PPS.

Validated manually on Wayland/Hyprland: 2256x1504 H.264 elementary
stream, ffprobe clean. Phase 3 marked complete in docs/PHASES.md.

docs/PHASES.md

@@ -32,9 +32,9 @@ frames.
 
 **Goal**: capture the desktop and feed frames into the encoder.
 
-- Add `libpipewire-0.3` dependency.
-- Implement `CaptureFactory::create()` using the xdg-desktop-portal.
-- Wire `CaptureSession::next_frame()` → `Encoder::encode()` in a local smoke
+- [x] Add `libpipewire-0.3` dependency.
+- [x] Implement `CaptureFactory::create()` using the xdg-desktop-portal.
+- [x] Wire `CaptureSession::next_frame()` → `Encoder::encode()` in a local smoke
   test that just writes a few encoded frames to disk.
 
 **Validation**: manual run on a real Linux desktop session produces a valid
@@ -89,4 +89,4 @@ where available.
 
 ## Current phase
 
-Phase 3 — PipeWire screen capture.
+Phase 4 — RTP Framing.