PlaceholderCoordinator

Drives a single shared 0f..1f highlight animation that every descendant placeholder reads from, so a list of skeleton items shimmers as one coordinated wave instead of each cell ticking on its own clock.

Obtain one via PlaceholderSurface (which also publishes it to LocalPlaceholderCoordinator). When a coordinator is active, the per-highlight animationSpec is ignored — the coordinator's spec governs every placeholder inside the scope.