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-core/src/config.rs

@@ -399,4 +399,27 @@ mod tests {
         c.fec.fec_percent = 15; // 250 + ceil(250*15/100)=288 > 255
         assert!(c.validate().is_err());
     }
+
+    #[test]
+    fn gamepad_pref_steam_roundtrip() {
+        use GamepadPref::*;
+        // Wire-byte round-trip for the Steam additions; an unknown byte still degrades to Auto.
+        for (p, b) in [(SteamController, 5u8), (SteamDeck, 6)] {
+            assert_eq!(p.to_u8(), b);
+            assert_eq!(GamepadPref::from_u8(b), p);
+        }
+        assert_eq!(GamepadPref::from_u8(99), Auto);
+        // Name parsing + canonical-name round-trip.
+        assert_eq!(GamepadPref::from_name("steamdeck"), Some(SteamDeck));
+        assert_eq!(GamepadPref::from_name("deck"), Some(SteamDeck));
+        assert_eq!(
+            GamepadPref::from_name("steamcontroller"),
+            Some(SteamController)
+        );
+        assert_eq!(SteamDeck.as_str(), "steamdeck");
+        assert_eq!(
+            GamepadPref::from_name(SteamController.as_str()),
+            Some(SteamController)
+        );
+    }
 }