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/host/mod.rs

@@ -1,5 +1,5 @@
 mod capture;
-mod tunnel;
+mod wayland;
 
 use anyhow::{Result, bail};
 use iroh::Endpoint;
@@ -71,14 +71,9 @@ async fn handle_peer(
 ) -> Result<()> {
     let (quic_send, quic_recv) = conn.accept_bi().await?;
 
-    // Spawn ffmpeg (and on Wayland, the gst-launch bridge) listening on a
-    // random localhost port. Returns the port and a guard that kills the
-    // child(ren) on drop.
     let capture_handle = capture::spawn(display, opts).await?;
     let port = capture_handle.local_port();
-
-    // Give ffmpeg's `-listen 1` HTTP server a moment to bind before we dial.
-    tokio::time::sleep(std::time::Duration::from_millis(150)).await;
+    wayland::wait_for_listener(port, std::time::Duration::from_secs(5)).await?;
     let tcp = TcpStream::connect(("127.0.0.1", port)).await?;
 
     let bridge = crate::common::tunnel::bridge(quic_send, quic_recv, tcp);
@@ -96,7 +91,7 @@ async fn handle_peer(
         }
     }
 
-    drop(capture_handle); // explicit teardown of ffmpeg/gst
+    capture_handle.shutdown().await;
     Ok(())
 }