Channel

A quantitative channel layered on a Facet's topology role: maps a numeric value read from each node onto a visual variable. Rides on the role — e.g. a connector edge's thickness or a node tile's opacity. v1 auto-scales linearly across the node set's observed [min, max]; an explicit scale domain/range can be added later. This is the open, general "channel" family — new domains add channels (numeric → visual variable), not new structural roles.