feat(host/steam): default the gadget Deck on for SteamOS (glass-confirmed)

The virtual Steam Deck is validated glass-to-glass on a Deck: it appears as a
distinct second Steam controller, a held A drives Steam's overlay ("Resume
Game"), and a button press registers in a real game (confirmed in-game).

gadget_preferred() now defaults ON for SteamOS hosts (/etc/os-release ID=steamos
or ID_LIKE), OFF elsewhere where the universal UHID path stays the default;
PUNKTFUNK_STEAM_GADGET=1/0 forces it. A Deck-as-host with a physical Deck never
reaches this path — resolve_gamepad's conflict gate degrades SteamDeck → DualSense
first, so the two-Deck case never happens in production (it was only a test-rig
confound on the dev Deck).

The feature is complete: a virtual Steam Deck that Steam Input recognizes +
promotes, churn-free, with input flowing to games. Workspace clippy/fmt/test
green. Not pushed.

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

crates/punktfunk-host/src/inject/linux/steam_controller.rs

@@ -384,8 +384,8 @@ impl SteamControllerManager {
         if idx >= MAX_PADS || self.pads[idx].is_some() || self.broken {
             return;
         }
-        // Prefer the USB gadget on SteamOS (the only transport Steam Input promotes); fall back to the
-        // universal UHID pad if the gadget is unavailable or not opted in.
+        // Prefer the USB gadget on SteamOS (default there — the only transport Steam Input promotes);
+        // fall back to the universal UHID pad if the gadget is unavailable or disabled.
         let opened = if crate::inject::steam_gadget::gadget_preferred() {
             crate::inject::steam_gadget::ensure_modules();
             match crate::inject::steam_gadget::SteamDeckGadget::open(idx as u8) {