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>

include/punktfunk_core.h

@@ -668,6 +668,44 @@ typedef struct {
 } PunktfunkRichInput;
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Forward-compatible superset of [`PunktfunkRichInput`] that can also express the rich Steam
+// surfaces: a *second* trackpad (`surface`), a distinct `click` vs touch, signed coordinates, and
+// pressure. Sent via [`punktfunk_connection_send_rich_input2`] — the only way a C client can emit a
+// `TouchpadEx`. The caller MUST set `struct_size = sizeof(PunktfunkRichInputEx)` (the ABI-skew
+// guard, like [`PunktfunkConfig`]); the legacy [`PunktfunkRichInput`] +
+// [`punktfunk_connection_send_rich_input`] stay byte-for-byte for existing callers.
+typedef struct {
+    // MUST equal `sizeof(PunktfunkRichInputEx)`.
+    uint32_t struct_size;
+    // One of `PUNKTFUNK_RICH_*` (`TOUCHPAD` / `MOTION` / `TOUCHPAD_EX`).
+    uint8_t kind;
+    // Gamepad index.
+    uint8_t pad;
+    // Touchpad/TouchpadEx: contact id.
+    uint8_t finger;
+    // Touchpad/TouchpadEx: 1 = finger down / touching, 0 = lifted.
+    uint8_t active;
+    // TouchpadEx: which surface — 0 = single/DualSense, 1 = Steam left pad, 2 = Steam right pad.
+    uint8_t surface;
+    // TouchpadEx: 1 = the pad is physically clicked (depressed), distinct from a touch contact.
+    uint8_t click;
+    // Reserved for alignment; set to 0.
+    uint8_t _reserved[2];
+    // TouchpadEx: x coordinate — **signed**, centred at 0 (the real Steam report convention). For a
+    // legacy `TOUCHPAD` kind sent through this struct, store the unsigned `0..=65535` value's bits.
+    int16_t x;
+    // TouchpadEx: y coordinate — signed, centred at 0.
+    int16_t y;
+    // TouchpadEx: contact pressure (`0` if the surface has no force sensor).
+    uint16_t pressure;
+    // Motion: gyro (pitch, yaw, roll), raw signed-16.
+    int16_t gyro[3];
+    // Motion: accelerometer (x, y, z), raw signed-16.
+    int16_t accel[3];
+} PunktfunkRichInputEx;
+#endif
+
 // A speed-test measurement, filled by [`punktfunk_connection_probe_result`]. `done` is 0 until
 // the host's end-of-burst report lands, then 1 (the numbers are final). `throughput_kbps` is the
 // delivered wire throughput to drive a bitrate choice from; `loss_pct` is the link loss and
@@ -1159,6 +1197,18 @@ PunktfunkStatus punktfunk_connection_send_rich_input(PunktfunkConnection *c,
                                                      const PunktfunkRichInput *rich);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Send a rich client→host input via the forward-compatible [`PunktfunkRichInputEx`] — the only way
+// a C client can emit a `TouchpadEx` (a second trackpad / signed coords / pressure). Set
+// `rich->struct_size = sizeof(PunktfunkRichInputEx)`; a smaller (older-layout) value is rejected.
+//
+// # Safety
+// `c` is a valid connection handle; `rich` is null or points to at least its declared
+// `struct_size` bytes.
+PunktfunkStatus punktfunk_connection_send_rich_input2(PunktfunkConnection *c,
+                                                      const PunktfunkRichInputEx *rich);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // The currently active session mode — the Welcome's, until an accepted
 // [`punktfunk_connection_request_mode`] switches it. Safe any time after connect.