Add XWayland support (#16)

* save dev state

* fix xwayland-satellite supervisor lifecycle, readiness and cleanup

* remove wayland scanner protocol generated files

* default xwayland enabled with path

* remove jargons

* address x11 review: park unconfirmed cleanup, never signal a reaped group, fix storm test and notify dir mode

* fix resize tests: never build Size values from negative dimensions

* add min-width/min-height window rules that replace client minimum sizes

* rename effective_size_constraints to effective_minimum_size and minor cleanups

docs/installation.md

@@ -54,6 +54,62 @@ other distributions; check the [Smithay
 dependencies](https://github.com/Smithay/smithay#system-dependencies) for equivalent
 libraries.
 
+### X11 support (optional)
+
+To run X11-only applications, Ferese launches
+[xwayland-satellite](https://github.com/Supreeeme/xwayland-satellite),
+which in turn needs an `Xwayland` binary. Both are runtime dependencies, not
+build dependencies, and neither is needed to build Ferese.
+
+- `Xwayland` ships with the X.Org server packages: `xorg-x11-server-Xwayland`
+  on Fedora and Debian/Ubuntu, `xorg-xwayland` on Arch.
+- `xwayland-satellite` is not packaged by most distributions. Build or install
+  it from its upstream releases **with its `systemd` feature enabled**
+  (`cargo build --release --features systemd`). That feature is what sends the
+  `READY=1` readiness notification Ferese waits for. Upstream's default feature
+  set is empty, so a default build never sends it, Ferese's fixed 10-second
+  startup budget expires, and the service reports a startup failure instead of
+  a display. A spawned process that is still alive is not readiness.
+
+Both the compositor's default and the packaged configuration ship with X11
+enabled, so a session with the two pieces above needs no configuration change.
+If you maintain your own `config.kdl`, the block to keep is
+
+```kdl
+xwayland {
+    enabled #true
+    startup  on-demand
+    path     "xwayland-satellite"
+}
+```
+
+in `~/.config/ferese/config.kdl` and starting a new session, then check
+`feresectl xwayland status`. See
+[X11 support](configuration.md#x11-support-xwayland-satellite) for the full key
+list, diagnostics, and known limitations.
+
+#### Sandboxed applications
+
+Ferese keeps the X11 cookie in a private file under `$XDG_RUNTIME_DIR`, and
+applications inherit it through `XAUTHORITY`. Sandboxed packaging formats do not
+all make that path visible, so an X11-only application may fail to start even
+though `feresectl xwayland status` reports a running service.
+
+Flatpak masks `/run/user/1000` with the application's own runtime directory, and
+a `fallback-x11` permission is not granted inside a Wayland session. Both effects
+deny the sandbox access to the display, so the application exits without a
+window. Grant the socket explicitly:
+
+```sh
+flatpak override --user --socket=x11 com.spotify.Client
+```
+
+Snaps under strict confinement usually expose the real `/run/user/1000`, so a
+snap that speaks X11 typically connects without extra configuration. That is a
+confinement detail rather than a guarantee: if a snap application still cannot
+reach the display, check whether its sandbox can read the `XAUTHORITY` path
+reported by `feresectl xwayland status`.
+
 ### Rust toolchain
 
 Install Rust through [rustup.rs](https://rustup.rs/). The checkout's