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/linux/dualshock4.rs

@@ -439,7 +439,9 @@ impl DualShock4Manager {
     /// pad isn't present.
     pub fn apply_rich(&mut self, rich: RichInput) {
         let idx = match rich {
-            RichInput::Touchpad { pad, .. } | RichInput::Motion { pad, .. } => pad as usize,
+            RichInput::Touchpad { pad, .. }
+            | RichInput::Motion { pad, .. }
+            | RichInput::TouchpadEx { pad, .. } => pad as usize,
         };
         if idx >= MAX_PADS || self.pads[idx].is_none() {
             return;
@@ -466,6 +468,26 @@ impl DualShock4Manager {
                 self.state[idx].gyro = gyro;
                 self.state[idx].accel = accel;
             }
+            RichInput::TouchpadEx {
+                surface,
+                finger,
+                touch,
+                x,
+                y,
+                ..
+            } => {
+                // A Steam right/single pad maps onto the one DS4 touchpad (signed centre-0 →
+                // 0..=65535); surface 1 (the Steam left pad) has no DS4 equivalent.
+                if surface != 1 {
+                    let slot = (finger as usize).min(1);
+                    let n = |v: i16| ((v as i32) + 32768) as u32;
+                    let t = &mut self.state[idx].touch[slot];
+                    t.active = touch;
+                    t.id = slot as u8;
+                    t.x = (n(x) * (DS4_TOUCH_W - 1) as u32 / u16::MAX as u32) as u16;
+                    t.y = (n(y) * (DS4_TOUCH_H - 1) as u32 / u16::MAX as u32) as u16;
+                }
+            }
         }
         self.write(idx);
     }