feat(host/steam): M2 — virtual Steam Deck as a wired PadBackend (Linux)

Make the virtual hid-steam device a selectable per-session host gamepad,
end-to-end on Linux: PUNKTFUNK_GAMEPAD=steamdeck now builds a
SteamControllerManager that creates a /dev/uhid 28DE:1205 Deck, enters
gamepad_mode, and feeds the byte-exact Deck report (M1).

- inject/linux/steam_controller.rs: SteamControllerManager / SteamDeckPad,
  mirroring dualsense.rs (open/create2, GET/SET_REPORT pump, heartbeat, RAII
  destroy). Two Steam-specific quirks beyond the DualSense path:
    * gamepad_mode entry — best-effort `lizard_mode=0` via sysfs, plus a b9.6
      creation pulse (MODE_ENTER) so steam_do_deck_input_event stops
      early-returning, plus an anti-toggle guard (MENU_HOLD_CAP) so a long
      in-game Start-hold can't flip gamepad_mode back off.
    * UHID_SET_REPORT answered err=0 (DualSense omits it; the kernel stalls
      ~5s/cmd otherwise); the 0xEB rumble report parsed onto the 0xCA plane.
- core config.rs: GamepadPref::SteamDeck (wire byte 6) + SteamController
  (byte 5, reserved — folds to Xbox360 until its backend lands); from_u8 /
  from_name / as_str. Forward-compatible (unknown byte -> Auto); the C-ABI
  PUNKTFUNK_GAMEPAD_* constants stay M3, so no generated-header drift.
- punktfunk1.rs: PadBackend::SteamDeck variant + select / handle / apply_rich
  / pump / heartbeat arms; pick_gamepad Linux arm.

On-box: an #[ignore]d backend test (backend_binds_and_input_flows) drives the
real SteamDeckPad — it binds hid-steam (gamepad + IMU evdevs), enters gamepad
mode, BTN_A reaches the evdev, and the device tears down on drop. Workspace
clippy/fmt/test green. Not pushed. Next: M3 (protocol/ABI wire) + M4 (client
capture).

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

crates/punktfunk-host/src/punktfunk1.rs

@@ -1399,6 +1399,8 @@ enum PadBackend {
     DualSense(crate::inject::dualsense::DualSenseManager),
     #[cfg(target_os = "linux")]
     DualShock4(crate::inject::dualshock4::DualShock4Manager),
+    #[cfg(target_os = "linux")]
+    SteamDeck(crate::inject::steam_controller::SteamControllerManager),
     #[cfg(target_os = "windows")]
     DualSenseWindows(crate::inject::dualsense_windows::DualSenseWindowsManager),
     #[cfg(target_os = "windows")]
@@ -1420,6 +1422,12 @@ impl PadBackend {
                 tracing::info!("gamepad backend: virtual DualShock 4 (UHID hid-playstation)");
                 return PadBackend::DualShock4(crate::inject::dualshock4::DualShock4Manager::new());
             }
+            GamepadPref::SteamDeck => {
+                tracing::info!("gamepad backend: virtual Steam Deck (UHID hid-steam)");
+                return PadBackend::SteamDeck(
+                    crate::inject::steam_controller::SteamControllerManager::new(),
+                );
+            }
             GamepadPref::XboxOne => {
                 tracing::info!("gamepad backend: uinput X-Box One/Series pad");
                 return PadBackend::Xbox360(crate::inject::gamepad::GamepadManager::with_identity(
@@ -1455,6 +1463,8 @@ impl PadBackend {
             PadBackend::DualSense(m) => m.handle(ev),
             #[cfg(target_os = "linux")]
             PadBackend::DualShock4(m) => m.handle(ev),
+            #[cfg(target_os = "linux")]
+            PadBackend::SteamDeck(m) => m.handle(ev),
             #[cfg(target_os = "windows")]
             PadBackend::DualSenseWindows(m) => m.handle(ev),
             #[cfg(target_os = "windows")]
@@ -1471,6 +1481,8 @@ impl PadBackend {
             PadBackend::DualSense(m) => m.apply_rich(_rich),
             #[cfg(target_os = "linux")]
             PadBackend::DualShock4(m) => m.apply_rich(_rich),
+            #[cfg(target_os = "linux")]
+            PadBackend::SteamDeck(m) => m.apply_rich(_rich),
             #[cfg(target_os = "windows")]
             PadBackend::DualSenseWindows(m) => m.apply_rich(_rich),
             #[cfg(target_os = "windows")]
@@ -1496,6 +1508,8 @@ impl PadBackend {
             PadBackend::DualSense(m) => m.pump(rumble, hidout),
             #[cfg(target_os = "linux")]
             PadBackend::DualShock4(m) => m.pump(rumble, hidout),
+            #[cfg(target_os = "linux")]
+            PadBackend::SteamDeck(m) => m.pump(rumble, hidout),
             #[cfg(target_os = "windows")]
             PadBackend::DualSenseWindows(m) => m.pump(rumble, hidout),
             #[cfg(target_os = "windows")]
@@ -1515,6 +1529,8 @@ impl PadBackend {
             PadBackend::DualSense(m) => m.heartbeat(std::time::Duration::from_millis(8)),
             #[cfg(target_os = "linux")]
             PadBackend::DualShock4(m) => m.heartbeat(std::time::Duration::from_millis(8)),
+            #[cfg(target_os = "linux")]
+            PadBackend::SteamDeck(m) => m.heartbeat(std::time::Duration::from_millis(8)),
             #[cfg(target_os = "windows")]
             PadBackend::DualSenseWindows(m) => m.heartbeat(std::time::Duration::from_millis(8)),
             #[cfg(target_os = "windows")]
@@ -1894,6 +1910,9 @@ fn pick_gamepad(pref: GamepadPref, env: Option<&str>, linux: bool, windows: bool
         // One/Series: a real, distinct uinput identity on Linux; folded into the 360 backend on
         // Windows (XInput can't tell them apart anyway).
         GamepadPref::XboxOne if linux => GamepadPref::XboxOne,
+        // Steam Deck: Linux UHID hid-steam. The classic Steam Controller's backend isn't built yet,
+        // so it folds to Xbox360 for now (Windows Steam devices are M7).
+        GamepadPref::SteamDeck if linux => GamepadPref::SteamDeck,
         _ => GamepadPref::Xbox360,
     }
 }