The active-side current vertex is red (primary) again; the green is
reserved for the go-indicators. A ring pulses briefly (SMIL animate
on r + opacity, keyed on side+vertex) each time the cursor lands,
to draw the eye — browsers won't let us move the OS mouse pointer.
https://claude.ai/code/session_01KNdXjQczMCRGMK7K3Qderw