Cut the active element out of the hint overlay

The overlay is now the same spotlight the tour draws: an svg dim with a
rounded cutout around the open hint's element (the path generator moves
out of the tour's context into a pure function both share). The element
stays visible and interactive inside the cutout — the svg lets pointer
events through and only the dimmed path catches them, so clicking the
dim still closes the hint.

The other beacons now wait under the overlay instead of floating above
it; only the open hint's beacon (via its aria-expanded state) and its
popover rise above. The cutout tracks the element on scroll and resize.

The arrow's drop-shadow halo is gone: it read as a dirty outline against
light pages — tours never needed one because the arrow sits against the
dark overlay, and hints now either touch the colored beacon (no overlay)
or sit against the dim (overlay), so the plain white triangle works in
both.