Applies the dark rounded-rect pillLabel treatment to labels that overlay
graph content across nlos.js scenes (knife-edge, earth curve, LOS, ray
trace, duct, duct types, real terrain). Fixes the "deep shadow" label
truncating off the right edge of the J(v) plot by right-aligning it to
the plot frame. The readout annotation now flips to the left side when
the cursor nears the right edge.
Adds a plain-language explanation of PWAT (precipitable water) in
why-signals-bend.md so readers don't have to already know the term.
Graham McIntire
a96d43cfbe