Clean up the hints module

show() now mounts hints whose elements have appeared since the last
call, which the docs already promised but only a full hide()/show()
delivered; mountHints() skips what is already on the page, making
show() naturally idempotent and simpler.

The closePopover()/close() pair collapses into one function, the
scroll and resize listeners use requireRefresh directly instead of
wrapper closures, and config/dismissed become consts.

Comments get a pass: the HintsConfig overlay comment still described
the pre-cutout semantics (beacons above the overlay) and is corrected;
obvious comments are dropped, the load-bearing ones tightened, and em
dashes across source comments replaced per Kamran's preference.