docs(demos): keep tour spotlight on its target while scrolling

[lesnik512]

On the demo pages, the guided-tour spotlight (dim panels, ring and coach card) stayed where it was on screen when you scrolled, so the bright hole drifted off its target.

The tour measured the targets with getBoundingClientRect() once per step and positioned position: fixed elements from that, with nothing listening for scroll or resize. makeTour now keeps the current step's target nodes and, while a step is open, recomputes the spotlight and coach position on scroll (capture, so nested scroll containers count too) and resize, at most once per animation frame. The listeners are removed when the step closes or the demo resets. All mount modes (single client, herd, full stack) share makeTour, so they all get the fix.

Re-placing the coach on scroll can switch which side it sits on more often, so the arrow's border overrides are now cleared before each placement. Otherwise borders from the previous side could carry over.

I considered switching the overlay to position: absolute in document coordinates. It would scroll with the page on its own, but the full-screen dim panels and the coach's viewport clamping would need reworking, so re-measuring is the smaller change.

Testing

The repo has no JS test harness, so I checked this with a throwaway headless Playwright script against mkdocs serve. It plays each demo until the first stop, scrolls 120px, and measures how far the ring moved:

engine	retry	retry herd	circuit breaker	full stack
main	0px	0px	0px	0px
this branch	120px	120px	120px	120px

mkdocs build --strict passes.