feat(host): X11 capture backend + shared pipeline extraction

Extract the display-agnostic encode/mux tail out of wayland.rs into a new
host/pipeline.rs: CaptureHandle + lifecycle, audio routing setup, the gst
arg builder, the spawn, and Serve::bind now live there. Backends supply
only their video-source element args plus a post-spawn hook (Wayland uses
it to close its leaked pipewire fd; X11 passes a no-op). capture.rs
collapses to a thin dispatcher; its CaptureHandle enum is gone.

Add host/x11.rs: ximagesrc (use-damage=false show-pointer=true), whole
root window by default or a single window via --window (xwininfo
click-picker → xid). x11rb reads geometry for an info log, justifying the
previously-vestigial dep. No portal, no fd dance — capture starts
silently when the first viewer connects (the ticket is the access
control). Viewer is display-agnostic and unchanged.

Wire --no-hwencode for real (was a no-op): the shared tail now selects
x264enc(tune=zerolatency,ultrafast)/I420 vs vah264enc/NV12 and switches
the videoconvert target format to match. Applies to both backends.

deps.rs: check_host_binaries now takes &HostOpts and checks shared
elements for both backends, encoder by --no-hwencode, source per backend
(pipewiresrc/ximagesrc), and xwininfo only when X11 + --window. Install
hints added for x264enc, ximagesrc, xwininfo.

Verified: warning-free build; smoke test still passes (tail unchanged);
ximagesrc + both encoder tails produce mpv-decodable H.264 against an
Xwayland root. Interactive cross-machine end-to-end pending.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/host/capture.rs

@@ -1,40 +1,20 @@
-//! 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.
+//! Capture dispatcher. Picks the per-display-server source backend and returns
+//! the shared [`pipeline::CaptureHandle`]. The handle itself, its teardown, and
+//! the whole encode/serve tail are display-agnostic and live in
+//! [`super::pipeline`]; the backends differ only in how they obtain a video
+//! source element.
 
-use anyhow::{Result, bail};
+use anyhow::Result;
 
 use crate::cli::HostOpts;
 use crate::common::display::DisplayServer;
-use crate::host::wayland;
-
-pub enum CaptureHandle {
-    Wayland(wayland::CaptureHandle),
-}
-
-impl CaptureHandle {
-    pub fn local_port(&self) -> u16 {
-        match self {
-            CaptureHandle::Wayland(h) => h.local_port(),
-        }
-    }
-
-    pub async fn shutdown(self) {
-        match self {
-            CaptureHandle::Wayland(h) => h.shutdown().await,
-        }
-    }
-}
+use crate::host::pipeline::CaptureHandle;
+use crate::host::{wayland, x11};
 
 pub async fn spawn(display: DisplayServer, opts: &HostOpts) -> Result<CaptureHandle> {
     match display {
-        DisplayServer::Wayland => {
-            let h = wayland::start(opts).await?;
-            Ok(CaptureHandle::Wayland(h))
-        }
-        DisplayServer::X11 => {
-            bail!("X11 capture pipeline not yet implemented (Phase 2 follow-up)");
-        }
+        DisplayServer::Wayland => wayland::start(opts).await,
+        DisplayServer::X11 => x11::start(opts).await,
         DisplayServer::Unknown => unreachable!("caller guarantees display != Unknown"),
     }
 }