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/capture.rs

@@ -1,43 +1,39 @@
-//! Capture-and-encode pipeline.
-//!
-//! Phase 1 stub: the actual ffmpeg / gst-launch spawn lives here. Returns a
-//! [`CaptureHandle`] whose `Drop` impl kills the children so a panic on the
-//! bridge side can't leave a black-hole encoder running.
-//!
-//! Phase 2 will fill in X11 (x11grab) and Wayland (ashpd portal +
-//! pipewiresrc → fdsink | ffmpeg) bodies; per-app PipeWire routing hangs off
-//! the Wayland branch.
+//! Capture dispatcher. Selects the per-display-server pipeline and returns its
+//! [`CaptureHandle`]. Each backend owns its own children and tears them down
+//! via its own Drop / shutdown.
 
 use anyhow::{Result, bail};
 
 use crate::cli::HostOpts;
 use crate::common::display::DisplayServer;
+use crate::host::wayland;
 
-pub struct CaptureHandle {
-    port: u16,
-    // Future: child handles, PipewireState, etc.
+pub enum CaptureHandle {
+    Wayland(wayland::CaptureHandle),
 }
 
 impl CaptureHandle {
     pub fn local_port(&self) -> u16 {
-        self.port
-    }
-}
-
-impl Drop for CaptureHandle {
-    fn drop(&mut self) {
-        // Future: SIGTERM children with a short grace period, then SIGKILL.
-        // Tear down any PipeWire null sinks / loopbacks created in spawn().
-    }
-}
-
-pub async fn spawn(display: DisplayServer, _opts: &HostOpts) -> Result<CaptureHandle> {
-    match display {
-        DisplayServer::X11 => {
-            bail!("X11 capture pipeline not yet implemented (Phase 2)");
+        match self {
+            CaptureHandle::Wayland(h) => h.local_port(),
         }
+    }
+
+    pub async fn shutdown(self) {
+        match self {
+            CaptureHandle::Wayland(h) => h.shutdown().await,
+        }
+    }
+}
+
+pub async fn spawn(display: DisplayServer, opts: &HostOpts) -> Result<CaptureHandle> {
+    match display {
         DisplayServer::Wayland => {
-            bail!("Wayland capture pipeline not yet implemented (Phase 2)");
+            let h = wayland::start(opts).await?;
+            Ok(CaptureHandle::Wayland(h))
+        }
+        DisplayServer::X11 => {
+            bail!("X11 capture pipeline not yet implemented (Phase 2 follow-up)");
         }
         DisplayServer::Unknown => unreachable!("caller guarantees display != Unknown"),
     }