fix(steamdeck): install the plugin runner + harden Deck pad typing + repair update.sh

- scripts/steamdeck/install.sh (§2b) + update.sh: build and install the
  scripting runner user-scoped (~/.local wrapper + pinned bun + bundle) —
  SteamOS's read-only /usr can't take the .deb layout, so the console's
  plugin store reported "the plugin runner isn't installed" on every
  SteamOS host. update.sh retrofits it onto existing installs.
- plugins.rs runner discovery: check the ~/.local layout after the /usr
  ones; mention the SteamOS path in the not-installed error.
- update.sh: define warn() — it was used but never defined, so under
  `set -e` the first warn aborted the update before services restarted.
- pf-client-core gamepad: a Steam Input virtual pad forwarded on a real
  Deck (the only-pad case) now declares the DECK kind in its per-pad
  arrival instead of the wrapper's Xbox 360 identity — the session-level
  auto_pref already resolved SteamDeck, but the arrival overrides it on
  current hosts.
- reject tests: cover SetupFailed; move the foreign-code probe off 0x68.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-client-core/src/gamepad.rs

@@ -876,10 +876,16 @@ impl Worker {
             );
             return;
         };
-        let pref = self
-            .pad_info(id)
-            .map(|p| p.pref)
-            .unwrap_or(GamepadPref::Xbox360);
+        let pref = match self.pad_info(id) {
+            // Steam Input's virtual pad standing in front of the Deck's built-in controls (the
+            // only-pad-forwarded case, [`Self::forwarded_ids`]): declare the DECK kind, not the
+            // wrapper's Xbox 360 identity. [`Self::auto_pref`] already resolves the SESSION
+            // default this way, but a current host honors the per-pad arrival over the session
+            // default — so without this the host builds an X-Box 360 pad on a real Deck.
+            Some(p) if p.steam_virtual && is_steam_deck() => GamepadPref::SteamDeck,
+            Some(p) => p.pref,
+            None => GamepadPref::Xbox360,
+        };
         match self.subsystem.open(sdl3::sys::joystick::SDL_JoystickID(id)) {
             Ok(pad) => {
                 let mut slot = Slot::new(id, index, pref, pad);