feat(host): fold Steam Deck to DualSense on Windows; split-pad touch + pad clicks

A Deck client asking for 'steamdeck' on a Windows host resolved to Xbox360,
whose apply_rich is a no-op — gyro and both trackpads silently discarded.
Windows already ships a full DualSense backend (UMDF driver, touchpad +
motion, wire units 1:1), so pick_gamepad now folds SteamDeck -> DualSense
there.

The three near-identical DualSense-family appliers (Linux UHID, Windows
DualSense, Windows DS4) are hoisted into one shared
dualsense_proto::DsState::apply_rich, with two mapping upgrades for Steam
dual-pad clients everywhere:
 * the Deck's two pads SPLIT the single DualSense touchpad — left pad ->
   contact 0 on the left half, right pad -> contact 1 on the right half —
   mirroring the physical thumb layout and the split-pad zones games and
   Steam Input already use (the left pad was previously dropped outright)
 * TouchpadEx pad clicks now press the touchpad-click button (persisted
   in DsState::touch_click, OR-ed in by both serializers; previously
   dropped by every DualSense-family backend, Linux included)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/inject/proto/dualshock4_proto.rs

@@ -53,7 +53,7 @@ pub fn serialize_state(r: &mut [u8; DS4_INPUT_REPORT_LEN], st: &DsState, counter
     r[4] = st.ry;
     r[5] = (st.dpad & 0x0F) | (st.buttons[0] & 0xF0); // dpad hat (low) + face buttons (high)
     r[6] = st.buttons[1]; // L1/R1, L2/R2 digital, Share/Options, L3/R3
-    r[7] = (st.buttons[2] & 0x03) | ((counter & 0x3F) << 2); // PS + touchpad-click + report counter
+    r[7] = (st.buttons2_with_click() & 0x03) | ((counter & 0x3F) << 2); // PS + touchpad-click (incl. rich pad clicks) + report counter
     r[8] = st.l2; // L2 analog (z)
     r[9] = st.r2; // R2 analog (rz)
     r[10..12].copy_from_slice(&ts.to_le_bytes()); // sensor_timestamp (struct off 9)