feat(gamepad): Windows Steam Deck backend — Steam-Input-promoted UMDF virtual Deck

The N4 GO verdict, productized. GamepadPref::SteamDeck on a Windows host
now builds a real virtual Deck instead of folding to DualSense: games
get native Deck glyphs + both trackpads + gyro + all four back grips
through Steam Input's own remapping.

- steam_deck_windows.rs: DeckWinPad/DeckWinProto/SteamDeckWindowsManager
  over the sealed shm channel, sharing the whole Linux Deck codec
  (steam_proto now compiles on Windows too — it was already pure). The
  SwDevice identity carries usb_mi: Some(2): the &MI_02 hardware-id
  token hidclass mirrors into the HID child and Steam parses as the
  wired controller interface — the promotion gate.
- Driver: DEVTYPE_STEAMDECK (3) graduates from the spike — SET_FEATURE
  0xEB rumble / 0x8F haptic pulses are republished to the host through
  the output slot (report-id-0 prefixed, so parse_steam_output sees the
  Linux wire shape), and the 0xAE/GET_STRING serial + 0x83 unit id are
  per-pad (read from the section's pad_index; PFDK<unit-id> matches
  steam_proto::deck_serial).
- Router: SteamDeck arms in the Windows Pads paths; pick_gamepad flips
  SteamDeck-if-windows -> SteamDeck (the DualSense fold retires);
  dualsense-windows-test grows --deck.

ON-GLASS VALIDATED on .173 (rebuilt signed driver 9.9.0714.12xx
installed, Steam live): the manager-created pad (index 1) enumerates
with per-pad serial PFDK50460001, Steam logs Interface: 2 ->
'!! Steam controller device opened' -> 'Steam Controller reserving
XInput slot 0' -> PollState 2 (actively polling our cycling input
frames) -> mapping activated; clean teardown on exit. Rumble round-trip
through a real game remains an on-glass debt (nothing sent 0xEB during
the idle hold).

Known gap vs Linux: no physical-Steam-controller conflict degrade on
Windows yet (degrade_steam_on_conflict is Linux-only — /sys scan); a
Windows equivalent needs SetupDi enumeration and is deferred.

Verified: .21 clippy -D warnings + 304/0 tests + fmt --all; .133 clippy
-D warnings + the WDK driver-workspace check.

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

crates/punktfunk-host/src/main.rs

@@ -429,9 +429,11 @@ fn real_main() -> Result<()> {
             let xbox = args.iter().any(|a| a == "--xbox");
             // `--edge` drives the DualSense Edge backend (device_type 2) and additionally holds
             // the R4/L4 paddles on the pressed beats, so a HID read shows the Edge bits in
-            // report byte 10 (0x80|0x40) next to Cross.
+            // report byte 10 (0x80|0x40) next to Cross. `--deck` drives the Steam Deck backend
+            // (device_type 3, the MI_02-promoted identity) — watch Steam claim it live.
             let edge = args.iter().any(|a| a == "--edge");
-            let extra_buttons: u32 = if edge {
+            let deck = args.iter().any(|a| a == "--deck");
+            let extra_buttons: u32 = if edge || deck {
                 punktfunk_core::input::gamepad::BTN_PADDLE1
                     | punktfunk_core::input::gamepad::BTN_PADDLE2
             } else {
@@ -535,6 +537,11 @@ fn real_main() -> Result<()> {
                     inject::dualsense_edge_windows::DualSenseEdgeWindowsManager::new(),
                     "DualSense Edge"
                 );
+            } else if deck {
+                drive!(
+                    inject::steam_deck_windows::SteamDeckWindowsManager::new(),
+                    "Steam Deck"
+                );
             } else {
                 drive!(
                     inject::dualsense_windows::DualSenseWindowsManager::new(),