v1.5.0: lint catches what spills out when zoomed in

Added

• Lint checks zoomed-in insides (minZoom): text must fit its box, an inside must not stick out of the element it
  sits in, insides of the same element must not overlap, and arrows among them must not pass through boxes.
• Lint warns when an arrow is drawn under the box it sits inside (and so is hidden), with the fix.
• layer is documented: drawing order from 0 (back) to 3 (front).
• examples/login-flow: a rich-visuals example from one prompt (hud theme, terminal, icons, zoom, chart, flow hand-offs), with a live demo and a README prompt row.

Changed

• Zoomed-in arrows draw on the box layer, so they show over the box they sit in instead of hiding under it.
• Example scenes must build with zero warnings; test/run.sh fails otherwise.

Fixed

• A flow packet's glow was clipped to a square when zoomed in.