The page can ask what is under a point

This release fills in hit-testing - asking which element sits under a given point on the page. `document.elementFromPoint(x, y)` and `elementsFromPoint(x, y)` were missing entirely, so drag-and-drop, tooltips, and custom controls that work out what you are pointing at hit a wall. They now answer for real, computed over the actual laid-out page: the first returns the topmost element at the point, the second the whole stack from top to bottom, a point over empty space resolves to the root as it does in every browser, and a point outside the window returns nothing - matching the reference browser on each case tested. Alongside it, three interface types are now present so the code that type-checks them stops throwing: MutationRecord (what a MutationObserver hands you), CustomElementRegistry, and IdleDeadline - all three back features the engine already runs. The engine gate holds at 6375 of 6375.