feat: touch input — TouchDown/Move/Up + host libei ei_touchscreen injection

Roadmap #5 (touch, ahead of the XL UHID DualSense work). Touch fits the existing 18-byte
InputEvent: code = touch id, x/y = client pixels, flags = (w<<16)|h — the same absolute
mapping as MouseMoveAbs.

- core: InputKind::{TouchDown=9, TouchMove=10, TouchUp=11} + from_u8 + roundtrip test.
- host inject/libei.rs: request the RemoteDesktop Touchscreen device type, bind the Touch
  capability, and inject ei_touchscreen down/motion/up (one event = one frame, per the
  protocol rule), mapping coordinates into the device region like the abs pointer. wlroots
  has no virtual-touch protocol wired — no-ops there.
- client-rs --touch-test: drags a synthetic finger (touch id 0) in a circle.

Validated live on headless KWin: the portal GRANTS the Touchscreen device type
(Keyboard|Pointer|Touchscreen), proving the request path — but KWin's EIS server creates no
touchscreen *device*, so touch currently no-ops on this KWin (now logged once, not silent).
The injection code is correct and will land on a backend that exposes ei_touchscreen
(gamescope / a newer compositor / the real touch-client path). Workspace green, clippy/fmt
clean, +1 unit test.

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

crates/punktfunk-core/src/input.rs

@@ -29,6 +29,14 @@ pub enum InputKind {
     /// Sticks are i16 range (−32768..32767) in the XInput/Moonlight convention — **+y =
     /// up** (unlike mouse coordinates); triggers 0..255.
     GamepadAxis = 8,
+    /// Touch begins. `code` = touch id (which finger; reusable after `TouchUp`), `x`/`y` =
+    /// pixel coordinates and `flags` = `(width << 16) | height` of the client's touch surface
+    /// — the same absolute mapping as [`MouseMoveAbs`](Self::MouseMoveAbs).
+    TouchDown = 9,
+    /// Touch moves. Same field meaning as [`TouchDown`](Self::TouchDown).
+    TouchMove = 10,
+    /// Touch ends. Only `code` (the touch id) is used.
+    TouchUp = 11,
 }
 
 /// The gamepad wire contract for [`InputKind::GamepadButton`]/[`InputKind::GamepadAxis`].
@@ -79,6 +87,9 @@ impl InputKind {
             6 => MouseScroll,
             7 => GamepadButton,
             8 => GamepadAxis,
+            9 => TouchDown,
+            10 => TouchMove,
+            11 => TouchUp,
             _ => return None,
         })
     }
@@ -148,4 +159,26 @@ mod tests {
         assert_eq!(InputEvent::decode(&e.encode()), Some(e));
         assert!(InputEvent::decode(&[0u8; INPUT_WIRE_LEN]).is_none()); // bad magic
     }
+
+    #[test]
+    fn touch_kinds_roundtrip() {
+        for kind in [
+            InputKind::TouchDown,
+            InputKind::TouchMove,
+            InputKind::TouchUp,
+        ] {
+            assert_eq!(InputKind::from_u8(kind as u8), Some(kind));
+            let e = InputEvent {
+                kind,
+                _pad: [0; 3],
+                code: 2, // touch id
+                x: 640,
+                y: 360,
+                flags: (1280u32 << 16) | 720, // client surface w/h
+            };
+            assert_eq!(InputEvent::decode(&e.encode()), Some(e));
+        }
+        // 12 (one past TouchUp) is not a valid kind.
+        assert_eq!(InputKind::from_u8(12), None);
+    }
 }