Add the hints module

A standalone hints() factory: a pulsing beacon per element that opens a
popover when clicked. Independent of the tour — no overlay, no body
class, no shared state — so the page stays interactive and hints can be
used without a tour at all.

The popover is the primitive extracted in the previous commit, anchored
to the beacon rather than the element, which keeps the arrow pointing at
the dot without any new positioning code.

Closing and dismissing are separate: an outside click, Escape or a second
click on the beacon closes the popover and keeps the beacon, while the
Got it button dismisses the hint. onDismiss plus stable ids is the
persistence hook; storage stays in userland.

Tour interplay needs no coupling in either direction. The tour already
marks the body while it runs, so CSS hides the beacons and a body-class
observer closes an open hint. Nothing to register, and it works with any
driver instance.

Beacons are real buttons (focus, Enter/Space, ARIA for free), centered on
one of the twelve anchor points of the element's box by CSS, sized and
coloured through custom properties. An IntersectionObserver hides a
beacon whose element scrolls out of view, and scroll is captured so
nested containers reposition too.