refactor(host/W6.2): extract the input-injection backends into the pf-inject crate

inject.rs + inject/* (the per-OS injectors — wlroots virtual-input, KWin
fake_input, libei/reis, gamescope-EI on Linux; SendInput on Windows — plus the
virtual-gamepad HID stack: DualSense/DualShock4/Switch Pro/Steam Controller/Deck
over uhid/usbip and the Windows UMDF drivers, the proto codecs, the injector
service, and the uhid manager) move into crates/pf-inject behind the
InputInjector trait (plan §W6). It consumes punktfunk_core::input (the neutral
GamepadEvent/InputEvent vocabulary, moved to core in W5) + the pf-driver-proto
wire contract, and reaches pf-capture only for the Windows gamepad-channel
WUDFHost check + the resident-mouse compose-kick hook.

The one inject->vdisplay coupling (the libei gamescope-EI backend needs the EIS
relay socket path) is broken via a leaf: gamescope_ei_socket_file moves to
pf-paths as the shared contract — the gamescope producer (host vdisplay) keeps
its session-env-lock wrapper around it, the libei consumer (pf-inject) reads it
directly post-retarget. The host keeps a `mod inject { pub use pf_inject::* }`
shim so every crate::inject::* path (the native/gamestream input planes + devtest)
is unchanged; the heavy input deps (wayland/reis/xkbcommon/usbip + the KWin
fake-input protocol XML) moved with the crate.

Verified: Linux clippy -D warnings (pf-inject + host nvenc,vulkan-encode,pyrowave
--all-targets) + pf-inject 69/69 + host 230/230 tests; Windows clippy -D warnings
(pf-inject --all-targets + host nvenc,amf-qsv --all-targets) Finished exit 0.

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

Cargo.lock

@@ -2880,6 +2880,34 @@ dependencies = [
 name = "pf-host-config"
 version = "0.12.0"
 
+[[package]]
+name = "pf-inject"
+version = "0.12.0"
+dependencies = [
+ "anyhow",
+ "ashpd",
+ "futures-util",
+ "libc",
+ "parking_lot",
+ "pf-capture",
+ "pf-driver-proto",
+ "pf-host-config",
+ "pf-paths",
+ "punktfunk-core",
+ "reis",
+ "tokio",
+ "tracing",
+ "usbip-sim",
+ "wayland-backend",
+ "wayland-client",
+ "wayland-protocols",
+ "wayland-protocols-misc",
+ "wayland-protocols-wlr",
+ "wayland-scanner",
+ "windows 0.62.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "xkbcommon",
+]
+
 [[package]]
 name = "pf-paths"
 version = "0.12.0"
@@ -3229,6 +3257,7 @@ dependencies = [
  "pf-frame",
  "pf-gpu",
  "pf-host-config",
+ "pf-inject",
  "pf-paths",
  "pf-win-display",
  "pf-zerocopy",