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>

Cargo.lock

@@ -932,6 +932,21 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "42703706b716c37f96a77aea830392ad231f44c9e9a67872fa5548707e11b11c"
 
+[[package]]
+name = "futures"
+version = "0.3.32"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b147ee9d1f6d097cef9ce628cd2ee62288d963e16fb287bd9286455b241382d"
+dependencies = [
+ "futures-channel",
+ "futures-core",
+ "futures-executor",
+ "futures-io",
+ "futures-sink",
+ "futures-task",
+ "futures-util",
+]
+
 [[package]]
 name = "futures-channel"
 version = "0.3.32"
@@ -939,6 +954,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "07bbe89c50d7a535e539b8c17bc0b49bdb77747034daa8087407d655f3f7cc1d"
 dependencies = [
  "futures-core",
+ "futures-sink",
 ]
 
 [[package]]
@@ -947,6 +963,17 @@ version = "0.3.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7e3450815272ef58cec6d564423f6e755e25379b217b0bc688e295ba24df6b1d"
 
+[[package]]
+name = "futures-executor"
+version = "0.3.32"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baf29c38818342a3b26b5b923639e7b1f4a61fc5e76102d4b1981c6dc7a7579d"
+dependencies = [
+ "futures-core",
+ "futures-task",
+ "futures-util",
+]
+
 [[package]]
 name = "futures-io"
 version = "0.3.32"
@@ -995,9 +1022,13 @@ version = "0.3.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "389ca41296e6190b48053de0321d02a77f32f8a5d2461dd38762c0593805c6d6"
 dependencies = [
+ "futures-channel",
  "futures-core",
+ "futures-io",
  "futures-macro",
+ "futures-sink",
  "futures-task",
+ "memchr",
  "pin-project-lite",
  "slab",
 ]
@@ -1468,6 +1499,7 @@ dependencies = [
  "axum-server",
  "cbc",
  "ffmpeg-next",
+ "futures-util",
  "hex",
  "libc",
  "lumen-core",
@@ -1476,6 +1508,7 @@ dependencies = [
  "pipewire",
  "rand 0.8.6",
  "rcgen",
+ "reis",
  "rsa",
  "rustls",
  "rustls-pemfile",
@@ -2172,6 +2205,19 @@ version = "0.8.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 
+[[package]]
+name = "reis"
+version = "0.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2aee09758db35e73eb1398c6ef321973ce7f4aad86573ba08c1c8ef98d350b46"
+dependencies = [
+ "enumflags2",
+ "futures",
+ "log",
+ "rustix",
+ "tokio",
+]
+
 [[package]]
 name = "ring"
 version = "0.17.14"