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>

implementations/Crafter.Graphics-Input.cpp

@@ -258,8 +258,9 @@ void Map::Attach(Window& w) {
     scrollListener = std::make_unique<EventListener<std::uint32_t>>(
         &w.onMouseScroll,
         [this](std::uint32_t delta) {
-            // Wayland and Win32 both deliver scroll as a signed delta
-            // packed into a uint32; we reinterpret to preserve sign.
+            // Every backend (DOM, Wayland, Win32) delivers scroll as
+            // signed whole detents (+1 = wheel down) packed into a
+            // uint32; we reinterpret to preserve sign.
             scrollAccumulator += (std::int32_t)delta;
         });
     lastMousePos.reset();