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/pf-driver-proto/src/lib.rs

@@ -538,10 +538,11 @@ pub mod gamepad {
     /// `device_type` = DualSense Edge (`VID_054C&PID_0DF2` HID identity — the DualSense report
     /// codec plus the four native back/Fn button bits).
     pub const DEVTYPE_DUALSENSE_EDGE: u8 = 2;
-    /// `device_type` = **N4-spike** Steam Deck identity (`VID_28DE&PID_1205`). Exists only for
-    /// the `deck-windows-spike` go/no-go probe (does Steam Input on Windows promote a
-    /// software-devnode HID Deck?) — never stamped by a session.
-    pub const DEVTYPE_STEAMDECK_SPIKE: u8 = 3;
+    /// `device_type` = Steam Deck controller (`VID_28DE&PID_1205` HID identity, the captured
+    /// controller-interface descriptor + the Steam `0x83`/`0xAE` feature contract). Promoted by
+    /// Steam Input on Windows when the devnode's synthesized USB hardware ids carry `&MI_02`
+    /// (the wired controller interface — the N4-spike finding).
+    pub const DEVTYPE_STEAMDECK: u8 = 3;
 
     /// The value a gamepad driver writes into its section's `driver_proto` field once it attaches —
     /// the host's positive "driver is alive on this section" signal (health check + version audit).