D58 — Impedance and width rules as synthetic net classes

Decision

Impedance and width rules become synthetic net classes (tmk_<RULE>_<ref>) for Default-class nets of instances at the apply threshold; used by the router only with --component-rules on; one width per net (the front-layer solution)

Alternatives

Per-layer widths; writing the classes into the project file

Reason

The router has one width per net, and the user's files are never written (rule 8). Board classes win; a class is never narrower than the board allows (doc 15 §15)