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>

include/punktfunk_core.h

@@ -201,6 +201,14 @@ enum PunktfunkInputKind
     // Sticks are i16 range (−32768..32767) in the XInput/Moonlight convention — **+y =
     // up** (unlike mouse coordinates); triggers 0..255.
     PUNKTFUNK_INPUT_KIND_GAMEPAD_AXIS = 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).
+    PUNKTFUNK_INPUT_KIND_TOUCH_DOWN = 9,
+    // Touch moves. Same field meaning as [`TouchDown`](Self::TouchDown).
+    PUNKTFUNK_INPUT_KIND_TOUCH_MOVE = 10,
+    // Touch ends. Only `code` (the touch id) is used.
+    PUNKTFUNK_INPUT_KIND_TOUCH_UP = 11,
 };
 #ifndef __cplusplus
 #if __STDC_VERSION__ >= 202311L