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>

README.md

@@ -16,8 +16,10 @@ without spinning up a Discord call or fighting with NAT.
 Working:
 - Wayland capture via the screencast portal (KDE Plasma 6 confirmed; other
   Wayland compositors with the portal should work but are untested)
+- X11 capture via `ximagesrc` (whole screen, or a single window with
+  `--window`); selected automatically, or forced with `--display-server x11`
 - VAAPI H.264 encode in GStreamer (RDNA3 confirmed; other VAAPI-capable
-  GPUs should work)
+  GPUs should work), with a software x264 fallback via `--no-hwencode`
 - Audio capture of the default sink's monitor, with optional per-app
   routing (`--app <name>`) and microphone mixing (`--mic`)
 - `--repair` cleanup of orphaned PipeWire state left by a crashed host
@@ -30,8 +32,10 @@ Working:
   `~/.config/pixelpass/config.toml` and used to auto-size the default
   viewer cap
 
-Not yet working:
-- X11 capture (stubbed, returns an error — Phase 2 follow-up)
+Not yet built (deferred, not blocking):
+- Per-monitor selection on a multi-monitor X11 host — `ximagesrc` grabs the
+  whole root canvas; single-monitor cropping needs xrandr region coords
+- `use-damage=true` CPU optimization for the X11 capture path
 
 ## Quick start
 
@@ -68,7 +72,7 @@ pixelpass <ticket>
 
 ## Requirements
 
-- Linux (Wayland session for now; X11 stubbed)
+- Linux (Wayland or X11; the backend is autodetected)
 - A VAAPI-capable GPU and the right driver:
   - AMD: `libva-mesa-driver`
   - Intel: `intel-media-driver` (modern iGPUs) or `intel-vaapi-driver` (older)
@@ -117,10 +121,10 @@ cargo build --release
 ```
   Host                                                          Viewer
   ────                                                          ──────
-  Wayland portal (ashpd) ──> PipeWire fd
-                              │
-                              ▼
-              gst-launch: pipewiresrc -> videorate -> vah264enc ->
+  Wayland portal (ashpd) ──> PipeWire fd ─┐   (X11: ximagesrc, no portal)
+                                          │
+                                          ▼
+              gst-launch: <source> -> videorate -> vah264enc/x264enc ->
                           h264parse -> mpegtsmux
                           (audio:    pulsesrc <sink>.monitor ->
                                      avenc_aac -> aacparse ─┘)
@@ -142,7 +146,7 @@ cargo build --release
 The viewer's player connects to a localhost HTTP server, which is
 just one end of the iroh tunnel. The host's HTTP server sits on the
 other end and streams GStreamer's stdout (an MPEG-TS containing
-hardware-encoded H.264 + AAC) through with no demux or remux.
+H.264 + AAC) through with no demux or remux.
 
 iroh handles NAT traversal: direct UDP if hole-punching succeeds,
 relay path otherwise. Both have been verified end-to-end.
@@ -201,6 +205,27 @@ If a host crashes mid-session it can leave orphaned `pixelpass_capture_*`
 null-sinks and their paired loopbacks loaded in PipeWire. Run
 `pixelpass --repair` to unload them and exit.
 
+## Display server
+
+The capture backend is autodetected from the environment
+(`WAYLAND_DISPLAY` → Wayland, else `DISPLAY` → X11, else
+`XDG_SESSION_TYPE`). Override it with `--display-server wayland|x11` — for
+example to force the X11 path while running inside a Wayland session (an
+Xwayland or Xephyr `DISPLAY`).
+
+- **Wayland** goes through the screencast portal: a "Share Screen?" dialog
+  appears when the first viewer connects, and you pick the monitor (or
+  window, with `--window`) there.
+- **X11** uses `ximagesrc` and starts silently when the first viewer
+  connects — the ticket is the access control, there's no portal gate.
+  `--window` runs an `xwininfo` picker (click the window you want to
+  share); without it the whole root window is captured.
+
+Encoding is hardware VAAPI (`vah264enc`) by default. `--no-hwencode`
+switches to software x264 (`x264enc tune=zerolatency`) for hosts without a
+working VAAPI H.264 entrypoint — higher CPU, no GPU needed. This applies
+to both backends.
+
 ## Multi-viewer
 
 One gst capture pipeline fans out to N concurrent viewers via a