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>

docs/roadmap.md

@@ -69,9 +69,14 @@ select = a `pw_stream` with `Direction::Output` + `media.class=Audio/Source`.
 
 ## 5. Touch + rich DualSense *(decision: commit to full UHID DualSense)*
 
-- **Touch (M):** `reis` already exposes `ei_touchscreen` — add Touch InputKinds + wire
-  `ei::Touchscreen` in `inject/libei.rs` (reuse the abs-pointer region mapping). Multi-touch
-  on KWin/Mutter; single-pointer fallback elsewhere.
+- **Touch — implemented (host path), pending a backend that lands it.** `TouchDown/Move/Up`
+  InputKinds (reuse the abs-pointer `flags=(w<<16)|h` mapping, `code`=touch id); host
+  `inject/libei.rs` requests the `Touchscreen` device type + binds the `Touch` capability and
+  injects `ei_touchscreen` down/motion/up; `punktfunk-client-rs --touch-test` drags a finger.
+  **Validated:** KWin's RemoteDesktop portal *grants* the Touchscreen device type, but its EIS
+  server creates **no touchscreen device** (headless KWin) — so touch currently no-ops on KWin
+  (now logged once). The code is correct; it needs a backend that exposes `ei_touchscreen`
+  (gamescope / newer KWin / the real iPad client path) to land. wlroots: no virtual-touch wired.
 - **Rich DualSense (XL, committed):** uinput can't carry HID output reports. Use **UHID** +
   the kernel `hid-playstation` driver (the inputtino/Wolf approach): present a genuine
   DualSense (real report descriptor, vendor 054C/0CE6, BT mode + CRC32) so games drive LED +