feat(input): wire native mouse-wheel scroll on Wayland + Win32, normalize all backends to ±1/detent (#32)

Window::onMouseScroll now fires on every backend and speaks one unit:
signed whole detents packed in the uint32 payload, +1 = wheel down
(DOM deltaY sign).

- Wayland: implement the previously-stubbed PointerListenerHandleAxis —
  vertical axis only, wl_fixed_to_double / 15 (libinput's units-per-
  detent), sub-detent remainder accumulated and reset on pointer leave.
- Win32: add the missing WM_MOUSEWHEEL case — -GET_WHEEL_DELTA_WPARAM /
  WHEEL_DELTA with a remainder accumulator for free-spinning wheels.
- DOM: dom-env.js normalizes WheelEvent.deltaY per deltaMode (100px /
  3 lines / 1 page per detent) with the same remainder scheme, so wasm
  delivers the identical unit instead of raw browser-specific deltas.
- Document the contract on Window::onMouseScroll.
- tests/MouseScroll: compositor-free regression test driving the
  Wayland axis handler directly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

additional/dom-env.js

@@ -472,7 +472,22 @@ function domAttachWindow(windowHandle) {
         }
         fire("__crafterDom_mouseUp", [e.button]);
     };
-    __windowListeners.wheel     = (e) => fire("__crafterDom_wheel",     [e.deltaY]);
+    // Window::onMouseScroll speaks whole wheel detents (+1 = wheel down),
+    // the same unit the Wayland and Win32 backends deliver. WheelEvent
+    // deltas arrive in browser-specific units, so normalize per deltaMode
+    // (Chromium: ~100px per detent in pixel mode; Firefox: 3 lines in line
+    // mode) and carry the sub-detent remainder so smooth-scroll trackpads
+    // add up instead of each event truncating to zero.
+    let __wheelDetentRemainder = 0;
+    __windowListeners.wheel = (e) => {
+        const perDetent = e.deltaMode === 1 ? 3 : e.deltaMode === 2 ? 1 : 100;
+        __wheelDetentRemainder += e.deltaY / perDetent;
+        const whole = Math.trunc(__wheelDetentRemainder);
+        if (whole !== 0) {
+            __wheelDetentRemainder -= whole;
+            fire("__crafterDom_wheel", [whole]);
+        }
+    };
     __windowListeners.contextmenu = (e) => { e.preventDefault(); };
     __windowListeners.pointerlockchange = () => {
         // Reset the synthetic accumulator when lock is released so the