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.

docs/RUNBOOK.md

@@ -27,6 +27,28 @@ FFmpeg may keep some "still reachable" allocations at exit; that is normal.
 This check caught two real defects in the Phase 2 decoder: a per-packet
 payload leak and an unpadded extradata buffer over-read. Keep using it.
 
+## Capture smoke test (Phase 3, manual)
+
+Requires a running desktop session with `xdg-desktop-portal` and a backend
+that implements the ScreenCast portal (e.g. Hyprland, GNOME, KDE). The
+portal shows an interactive source picker, so this cannot run unattended.
+
+```sh
+meson compile -C build                       # builds tools/capture_smoke
+./build/tools/capture_smoke 10 out.h264      # captures 10 frames
+ffprobe -v error -show_entries stream=codec_name,width,height out.h264
+```
+
+Expected: the tool prints the negotiated resolution/format/stride and
+writes a non-empty file; `ffprobe` reports `codec_name=h264` and the correct
+width/height. The encoder prepends its Annex-B SPS/PPS extradata so the
+file is a self-contained elementary stream.
+
+If you see `impl_ext_end_proxy called from wrong context` warnings, a
+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`.
+
 ## Formatting
 
 ```sh