Point the hint arrow at the beacon, resize the dot, add an optional overlay

Three fixes to how hints look and behave:

The popover arrow used to land beside the dot: it sits a fixed corner
inset from the popover's edge, while start/end alignment put the popover
edge flush with the beacon. The position options now pass the difference
between the arrow tip and the beacon's half-size as the alignment
pull-back, so the arrow points at the beacon's center. The arrow also
gets a faint halo — it is a white triangle designed to sit against a
tour's darkened overlay, and vanished on light pages without one.

The default beacon grows from 20px to 24px (still --driver-hint-size),
and defaults to a lighter indigo.

New opt-in overlay: overlay: true dims the page while a hint is open
(overlayColor/overlayOpacity to taste). Beacons and the popover sit
above it so hints stay clickable, and clicking the dimmed page closes
the hint like any outside click. Verified in a real browser — beacons
float above the dim, arrow centered on the dot.