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/input.rs

@@ -66,10 +66,24 @@ pub mod gamepad {
     pub const BTN_B: u32 = 0x2000;
     pub const BTN_X: u32 = 0x4000;
     pub const BTN_Y: u32 = 0x8000;
+    // Extended buttons in Moonlight's `buttonFlags2 << 16` namespace (see `gamestream/gamepad.rs`),
+    // so the GameStream paddle path and the native path share one host injector map. The four Steam
+    // Deck back grips (L4/L5/R4/R5) reuse the four GameStream/Xbox-Elite paddle slots — a semantic
+    // 1:1 for binding (the device identity carries the glyph distinction).
+    /// Back grip R4 — SDL `RightPaddle1` / GameStream `PADDLE1`.
+    pub const BTN_PADDLE1: u32 = 0x0001_0000;
+    /// Back grip L4 — SDL `LeftPaddle1` / GameStream `PADDLE2`.
+    pub const BTN_PADDLE2: u32 = 0x0002_0000;
+    /// Back grip R5 — SDL `RightPaddle2` / GameStream `PADDLE3`.
+    pub const BTN_PADDLE3: u32 = 0x0004_0000;
+    /// Back grip L5 — SDL `LeftPaddle2` / GameStream `PADDLE4`.
+    pub const BTN_PADDLE4: u32 = 0x0008_0000;
     /// DualSense touchpad click. Moonlight's extended-button position (`buttonFlags2`
     /// merges in at `<< 16`, see `gamestream/gamepad.rs`), so GameStream clients land on
     /// the same bit. Only the DualSense backend renders it; the xpad has no such button.
     pub const BTN_TOUCHPAD: u32 = 0x10_0000;
+    /// Misc / capture button — the Deck `…`/quick-access, Share/Capture / GameStream `MISC`.
+    pub const BTN_MISC1: u32 = 0x0020_0000;
 
     /// Axis ids for `InputKind::GamepadAxis`.
     pub const AXIS_LS_X: u32 = 0;