feat(host/steam): raw_gadget Deck host backend (Steam-Input path, opt-in)

Port the proven raw_gadget virtual Deck to a Rust host gamepad backend, the
SteamOS-only transport that gets Steam Input to actually promote the Deck.

- inject/linux/steam_gadget.rs (new): SteamDeckGadget — a userspace raw_gadget
  emulator of the real 3-interface USB Deck (mouse=0/keyboard=1/controller=2,
  28DE:1205) on a dummy_hcd loopback UDC, descriptors captured from a physical
  Deck, answering every control transfer incl. the HID feature reports. Driven by
  the same steam_proto::serialize_deck_state as the UHID pad; rumble feedback via
  parse_steam_output. The raw_gadget UAPI is funneled through 4 documented ioctl
  wrappers (the crate denies undocumented unsafe).
- inject/linux/steam_controller.rs: the manager pad is now a DeckTransport enum
  (Uhid | Gadget); ensure() prefers the gadget when PUNKTFUNK_STEAM_GADGET=1
  (best-effort modprobe dummy_hcd+raw_gadget), gracefully falling back to the
  universal UHID SteamDeckPad. write/pump/heartbeat dispatch through the enum.

Validated on a real Deck via a static musl harness that #[path]-includes the
module: enumerates, hid-steam binds + reads our serial + creates the Steam Deck +
Motion Sensors evdevs — identical to the C PoC. Caught a real portability bug:
raw_gadget's no-arg ioctls (RUN/CONFIGURE/EP0_STALL) reject a non-zero `value`
with EINVAL, and on musl an omitted ioctl vararg is a garbage register — so they
must pass an explicit 0.

Opt-in (default off) while the Steam GetControllerInfo feature contract is
hardened (to stop the gamepad-evdev churn). Workspace clippy/fmt/test green. Not pushed.

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

crates/punktfunk-host/src/inject.rs

@@ -495,6 +495,12 @@ mod gamepad_raii;
 #[cfg(target_os = "linux")]
 #[path = "inject/linux/steam_controller.rs"]
 pub mod steam_controller;
+/// Linux: virtual Steam Deck via the USB gadget subsystem (`raw_gadget` + `dummy_hcd`) — the only
+/// virtual-Deck transport Steam Input promotes (presents the controller on USB interface 2).
+/// SteamOS-host only (needs `dummy_hcd` + `raw_gadget`).
+#[cfg(target_os = "linux")]
+#[path = "inject/linux/steam_gadget.rs"]
+pub mod steam_gadget;
 /// Transport-independent Steam Controller / Steam Deck HID contract (descriptor, byte-exact Deck
 /// serializer, XInput/rich mappers, rumble parser), used by the Linux UHID backend ([`steam_controller`]).
 #[cfg(target_os = "linux")]