🐞 Stop trackpad gestures from scrolling the page behind them

[mrkai77]

With the native macOS gestures turned off, 3 and 4-finger Loop gestures would also scroll the page underneath, keep scrolling after lifting your fingers, and even trigger back/forward in browsers. This is because macOS turns unassigned 3 and 4-finger swipes into scrolls, and the gesture blocker only kicked in once Loop had recognized the gesture. By then, the page had already started scrolling, and the momentum carried on after the lift. This PR:

1. Holds scrolls that could belong to a Loop gesture from began while Loop decides on the stroke. If Loop takes it, the scroll is dropped. Otherwise, the held events are replayed in order through the tap proxy,
2. Lets swipes in unbound directions pass straight through, so they still scroll with no delay,
3. Decides momentum once, when it begins, so a stroke Loop took no longer flings the page after you lift your fingers,
4. Holds and replays navigation swipes (kIOHIDEventTypeNavigationSwipe, aka what browsers use for back/forward) the same way as scrolls,
5. Blocks rotation, zoom and smart zoom while a Loop gesture is active. Touch frames, force clicks and gesture brackets still pass, so apps don't get stuck mid-gesture,
6. Adds scrollOwner(heldFor:direction:) to SystemGestureFilter so the blocker can tell whether Loop owns the current touch. Loop keeps ownership until every finger lifts, since they rarely leave at the same time.