feat(steam): M4 complete — C-ABI send path, Decky UX, Apple/Android parity

Finish the client side of the Steam Controller / Steam Deck pipeline.

- C-ABI (core abi.rs): PunktfunkRichInputEx — a size-prefixed superset of
  PunktfunkRichInput that can express the second trackpad (surface), a distinct
  click vs touch, signed coords + pressure — plus
  punktfunk_connection_send_rich_input2 (the struct_size ABI-skew-guard
  precedent). The only way a C client (Apple/embedders) can emit a TouchpadEx;
  the legacy struct + send_rich_input stay byte-for-byte. punktfunk_core.h
  regenerated.

- Decky (clients/decky): a "Steam Deck" gamepad type in Settings + an unmissable
  Disable-Steam-Input instruction shown when it's selected (in Game Mode Steam
  Input holds 0x1205, so the SDL HIDAPI Steam driver can't open the Deck's
  controls until the user disables Steam Input for the shortcut). Plus a
  best-effort, feature-detected disableSteamInputForShortcut() in launchStream —
  never blocks/throws; the manual toggle is the documented source of truth.

- Apple parity (PunktfunkConnection.swift): GamepadType.steamController/steamDeck
  (wire 5/6) + name parsing, so the resolved type round-trips. Capture is blocked
  (GameController never surfaces a 0x28DE HID device).

- Android parity (Gamepad.kt): PREF_STEAMCONTROLLER/STEAMDECK + the Valve 0x28DE
  PIDs in prefFor(). Rich-input capture stays out of scope (no rich-input plane
  yet) — standard buttons/sticks resolve to the host's Steam Deck pad.

Rust workspace clippy/fmt/test green; Decky src/ typechecks clean (only a
pre-existing @decky/api dep resolution error remains); Swift/Kotlin compile on
their CI. The full pipeline is now BUILT; what remains is validation that needs
hardware we don't have (a running Steam on the host, a live Deck client, the
Moonlight paddle regression). Not pushed.

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

clients/decky/src/index.tsx

@@ -294,7 +294,13 @@ const RESOLUTIONS: [number, number, string][] = [
   [2560, 1440, "2560 × 1440"],
 ];
 const REFRESH = [0, 30, 60, 90, 120];
-const GAMEPADS = ["auto", "xbox360", "dualsense"];
+const GAMEPADS = ["auto", "xbox360", "dualsense", "steamdeck"];
+const GAMEPAD_LABELS: Record<string, string> = {
+  auto: "Automatic",
+  xbox360: "Xbox 360",
+  dualsense: "DualSense",
+  steamdeck: "Steam Deck",
+};
 
 const SettingsSection: FC = () => {
   const [s, setS] = useState<StreamSettings | null>(null);
@@ -355,14 +361,17 @@ const SettingsSection: FC = () => {
       />
       <Field label="Gamepad type" childrenContainerWidth="max">
         <Dropdown
-          rgOptions={GAMEPADS.map((g) => ({
-            data: g,
-            label: g === "auto" ? "Automatic" : g === "xbox360" ? "Xbox 360" : "DualSense",
-          }))}
+          rgOptions={GAMEPADS.map((g) => ({ data: g, label: GAMEPAD_LABELS[g] ?? g }))}
           selectedOption={s.gamepad}
           onChange={(o) => patch({ gamepad: o.data as string })}
         />
       </Field>
+      {s.gamepad === "steamdeck" && (
+        <Field
+          label="⚠ Disable Steam Input"
+          description="Steam Deck mode forwards the paddles, both trackpads, and gyro to the host. For that, Steam Input must be OFF for punktfunk: on the game page tap ⚙ → Controller Settings → set Steam Input to Off. Otherwise Steam keeps the Deck's controls and only the sticks + buttons reach the host."
+        />
+      )}
       <ToggleField
         label="Stream microphone"
         checked={s.mic_enabled}