feat: layers.modalOpen store and hover UI that never opens under a modal

Atlas app: an edge-peek panel opened under a modal dialog. The layer engine already knows when a modal is open (onLayerChange), but exposes it only as a DOM event, and the lib's own hover-opened UI (Tooltip, HoverCard, NavigationMenu, DropdownMenu submenus) does not consult it.

Plan: reactive layers store (layers.modalOpen, layers.count) from primitives/layer; Tooltip, HoverCard, NavigationMenuItem, DropdownMenuSub hover paths never open while a modal layer is open unless the trigger is inside that modal. Apps gate their own hover/edge UI with the same store.