Wayland capture: ashpd portal → gst-launch → ffmpeg → MPEG-TS

Implements the Wayland host pipeline from plan §4.5:

  ashpd ScreenCast portal
    -> CreateSession + SelectSources + Start + OpenPipeWireRemote
    -> (pipewire fd, node_id, width, height)
  gst-launch-1.0 pipewiresrc fd=N path=NODE_ID ! videoconvert
    ! video/x-raw,format=NV12 ! fdsink fd=1
  ffmpeg
    -f rawvideo -pix_fmt nv12 -video_size WxH -i pipe:0
    -f pulse -i default
    -c:v libx264 -preset ultrafast -tune zerolatency
    -c:a aac -f mpegts -listen 1 http://127.0.0.1:<rand>

Phase 1 ships software x264 per plan §7; VAAPI is Phase 2.

src/host/wayland.rs is the new module. capture.rs becomes a thin
dispatcher with a CaptureHandle enum (Wayland today, X11 next).
host/mod.rs swaps the 150ms sleep for a poll-until-listener-ready
helper, and calls handle.shutdown().await for an orderly SIGTERM /
1s grace / SIGKILL teardown. The Drop impl is the panic backstop.

The pipewire fd handoff clears CLOEXEC before gst-launch spawn and
closes the parent's copy of the raw fd after the child has it.

Also deletes the empty src/host/tunnel.rs and src/viewer/tunnel.rs
placeholder files — the generic bridge in common/tunnel.rs is doing
the work, and there's no host- or viewer-specific tunnel concern
worth a module yet.

src/viewer/mod.rs

@@ -1,5 +1,3 @@
-mod tunnel;
-
 use anyhow::Result;
 use iroh::Endpoint;
 use iroh::endpoint::presets;