feat: M2 P1.7 — libei input backend (portable to KWin/GNOME)

Add a second input-injection backend that works on compositors implementing
the org.freedesktop.portal.RemoteDesktop interface (KWin, GNOME/Mutter), where
the wlroots virtual-input protocols are absent. Uses ashpd 0.13 to open a
RemoteDesktop session + EIS fd and reis 0.6.1 to drive it as an EI sender:
bind pointer/keyboard/scroll/button capabilities and, per device,
start_emulating → emit → frame. Runs on a dedicated thread with its own tokio
runtime (the portal session + EIS connection must stay alive and the event
stream must be polled continuously); open() returns immediately so a slow or
denied portal can never freeze the ENet control thread, with events enqueued
over an unbounded channel until devices resume.

Backend now auto-selects per session (inject::default_backend): wlr on Sway,
libei on KDE/GNOME; LUMEN_INPUT_BACKEND overrides. Refactor inject.rs into the
inject/{wlr,libei}.rs layout matching the capture/encode convention. Keyboard
codes are evdev (the same space our VK→evdev table produces) and the compositor
supplies the keymap, so no keymap upload and no modifier serialization — pressing
the modifier keys Moonlight sends is enough.

Add a `lumen-host input-test` subcommand that injects a scripted mouse+keyboard
pattern through the session backend, so input injection can be validated without
a Moonlight client.

Live-validated on headless KWin (Plasma 6.4): mouse motion, left click, and the
'A' key inject correctly and are delivered to the focused client.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/lumen-host/Cargo.toml

@@ -31,10 +31,11 @@ rustls-pemfile = "2"
 rusty_enet = "0.4"
 
 [target.'cfg(target_os = "linux")'.dependencies]
-# `screencast` gates the ScreenCast portal module; `tokio` is the default runtime.
+# `screencast` gates the ScreenCast portal module; `remote_desktop` adds the RemoteDesktop
+# portal we use for libei input on KWin/GNOME; `tokio` is the default runtime.
 # `open_pipe_wire_remote` is unconditional, so ashpd's own `pipewire` feature is not
 # needed — we drive PipeWire with the `pipewire` crate below.
-ashpd = { version = "0.13", features = ["screencast"] }
+ashpd = { version = "0.13", features = ["screencast", "remote_desktop"] }
 ffmpeg-next = "7"
 libc = "0.2"
 # Must match the pipewire crate ashpd 0.13 links (libspa/pipewire-sys `links` key is
@@ -52,3 +53,8 @@ wayland-protocols-misc = { version = "0.3", features = ["client"] }
 xkbcommon = "0.8"
 # Opus encode for the GameStream audio stream (links system libopus).
 opus = "0.3"
+# libei (EI sender) for the portable input path on KWin/GNOME (RemoteDesktop portal).
+# The `tokio` feature wires reis's event stream into tokio's reactor.
+reis = { version = "0.6.1", features = ["tokio"] }
+# `StreamExt::next` on reis's tokio event stream in the libei worker loop.
+futures-util = "0.3"