feat(proto/steam): M3 — rich Steam wire (back buttons + 2nd trackpad)

Carry the rich Steam Controller / Steam Deck inputs end-to-end on the wire —
strictly additive + forward-compatible (unknown kinds/bits drop on old peers).

Core (punktfunk-core):
- input.rs: BTN_PADDLE1..4 + BTN_MISC1 in Moonlight's buttonFlags2<<16 namespace
  (so the GameStream paddle path and native grips share one host injector map;
  Steam L4/L5/R4/R5 reuse the four Xbox-Elite paddle slots).
- quic.rs: RichInput::TouchpadEx (kind 0x03 — surface 0/1/2, touch+click, signed
  coords, pressure; the second trackpad the single Touchpad can't express) and
  HidOutput::TrackpadHaptic (kind 0x04 — the SC voice-coil pulse). Round-tripped.
- abi.rs: PUNKTFUNK_GAMEPAD_STEAMDECK=6 / _STEAMCONTROLLER=5, the paddle bits,
  RICH_TOUCHPAD_EX / HIDOUT_TRACKPAD_HAPTIC constants. from_hid packs
  TrackpadHaptic into the existing which + effect[0..6] — the legacy structs do
  NOT grow (guarded by new size_of==20/19 asserts); GamepadPref lockstep +
  paddle-bit lockstep asserts extended. include/punktfunk_core.h regenerated.

Host (punktfunk-host):
- steam_proto::from_gamepad maps the wire paddles -> the four Deck grips + QAM;
  apply_rich routes TouchpadEx left/right -> the matching pad.
- every DualSense/DS4 manager (Linux + Windows) gained a TouchpadEx arm
  (surface 0/2 -> its one touchpad; surface 1 ignored) so the variant compiles
  everywhere and a Steam client streaming to a DS host keeps its right pad.
- the xpad BUTTON_MAP finally consumes the GameStream paddle bits
  (BTN_TRIGGER_HAPPY5-8) — Sunshine/Moonlight paddle clients were silently
  no-op'd before (design §5.6).
- Android feedback: drop TrackpadHaptic (no coils; rumble rides 0xCA).

Validated on-box: the ignored backend test now drives the full wire path —
from_gamepad (BTN_A + the L4 grip) + apply_rich (a left-pad TouchpadEx) reach the
evdev as BTN_A + ABS_HAT0X=-8000. Wire round-trips + paddle/TouchpadEx mapping
unit-tested. Workspace clippy/fmt/test green. Not pushed.

Deferred to M4: the C-ABI PunktfunkRichInputEx + send_rich_input2 (only the
Apple/embedder *send* path needs it; the host decodes TouchpadEx today).

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

crates/punktfunk-host/src/inject/proto/steam_proto.rs

@@ -221,6 +221,13 @@ impl SteamState {
         // The DualSense touchpad-click wire bit maps to the Deck's RIGHT pad click (the pad that
         // stands in for the DualSense touchpad — see apply_rich).
         set(&mut b, on(gs::BTN_TOUCHPAD), btn::RPAD_CLICK);
+        // Back grips (the whole reason for the Deck identity): the wire paddle bits map to the four
+        // Deck grips — PADDLE1/2/3/4 = R4/L4/R5/L5 (see `input::gamepad`); MISC1 = the QAM '…' button.
+        set(&mut b, on(gs::BTN_PADDLE1), btn::R4);
+        set(&mut b, on(gs::BTN_PADDLE2), btn::L4);
+        set(&mut b, on(gs::BTN_PADDLE3), btn::R5);
+        set(&mut b, on(gs::BTN_PADDLE4), btn::L5);
+        set(&mut b, on(gs::BTN_MISC1), btn::QAM);
         s.buttons = b;
         s
     }
@@ -241,6 +248,28 @@ impl SteamState {
                 self.gyro = gyro;
                 self.accel = accel;
             }
+            RichInput::TouchpadEx {
+                surface,
+                touch,
+                click,
+                x,
+                y,
+                ..
+            } => {
+                // Steam pads are natively signed (centre 0), so x/y map straight in. surface 1 =
+                // left pad, anything else (0 single / 2 right) = right pad.
+                if surface == 1 {
+                    self.press(btn::LPAD_TOUCH, touch);
+                    self.press(btn::LPAD_CLICK, click);
+                    self.lpad_x = x;
+                    self.lpad_y = y;
+                } else {
+                    self.press(btn::RPAD_TOUCH, touch);
+                    self.press(btn::RPAD_CLICK, click);
+                    self.rpad_x = x;
+                    self.rpad_y = y;
+                }
+            }
         }
     }
 }
@@ -424,6 +453,53 @@ mod tests {
         assert_eq!(s.accel, [4, 5, 6]);
     }
 
+    /// M3: the wire back-button bits map to the four Deck grips + QAM, and `TouchpadEx` routes the
+    /// left / right surfaces to the matching pad (signed coords pass straight through).
+    #[test]
+    fn back_buttons_and_dual_trackpad_mapping() {
+        let s = SteamState::from_gamepad(
+            gs::BTN_PADDLE1 | gs::BTN_PADDLE2 | gs::BTN_PADDLE3 | gs::BTN_PADDLE4 | gs::BTN_MISC1,
+            0,
+            0,
+            0,
+            0,
+            0,
+            0,
+        );
+        assert_ne!(s.buttons & btn::R4, 0); // PADDLE1 = R4
+        assert_ne!(s.buttons & btn::L4, 0); // PADDLE2 = L4
+        assert_ne!(s.buttons & btn::R5, 0); // PADDLE3 = R5
+        assert_ne!(s.buttons & btn::L5, 0); // PADDLE4 = L5
+        assert_ne!(s.buttons & btn::QAM, 0); // MISC1 = QAM
+
+        let mut s = SteamState::neutral();
+        s.apply_rich(RichInput::TouchpadEx {
+            pad: 0,
+            surface: 1,
+            finger: 0,
+            touch: true,
+            click: true,
+            x: -5000,
+            y: 6000,
+            pressure: 100,
+        });
+        assert_ne!(s.buttons & btn::LPAD_TOUCH, 0);
+        assert_ne!(s.buttons & btn::LPAD_CLICK, 0);
+        assert_eq!((s.lpad_x, s.lpad_y), (-5000, 6000));
+        s.apply_rich(RichInput::TouchpadEx {
+            pad: 0,
+            surface: 2,
+            finger: 0,
+            touch: true,
+            click: false,
+            x: 7000,
+            y: -8000,
+            pressure: 0,
+        });
+        assert_ne!(s.buttons & btn::RPAD_TOUCH, 0);
+        assert_eq!((s.rpad_x, s.rpad_y), (7000, -8000));
+    }
+
     /// The serial reply carries the leading report-id byte the kernel strips, so the *stripped*
     /// view (`reply[1..]`) is what `steam_get_serial` validates: `[0xAE, len, 0x01, ascii…]`.
     #[test]