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/mod.rs

@@ -1,7 +1,9 @@
 pub mod audio;
 mod capture;
+mod pipeline;
 mod serve;
 mod wayland;
+mod x11;
 
 use anyhow::{Result, bail};
 use iroh::endpoint::{Connection, presets};
@@ -17,7 +19,7 @@ use crate::common::{
     tunnel,
 };
 
-use self::capture::CaptureHandle;
+use self::pipeline::CaptureHandle;
 
 /// Messages from per-viewer tasks to the capture supervisor.
 enum SupervisorMsg {
@@ -32,7 +34,7 @@ enum SupervisorMsg {
 
 pub async fn run(opts: HostOpts) -> Result<()> {
     let display = DisplayServer::resolve(opts.display_server);
-    deps::check_host_binaries(display)?;
+    deps::check_host_binaries(display, &opts)?;
 
     if display == DisplayServer::Unknown {
         bail!(
@@ -249,7 +251,7 @@ fn print_host_banner(
     eprintln!("│  display server : {display:?}");
     eprintln!("│  capture        : {}", capture_summary(opts));
     eprintln!("│  bitrate / fps  : {} kbps @ {} fps", opts.bitrate, opts.framerate);
-    eprintln!("│  hw encode      : {}", if opts.no_hwencode { "off" } else { "auto (VAAPI if available)" });
+    eprintln!("│  hw encode      : {}", if opts.no_hwencode { "off (software x264)" } else { "on (VAAPI H.264)" });
     eprintln!("│  max viewers    : {} ({})", resolution.value, resolution.source.label());
     eprintln!("│");
     if clipboard_ok {