D76 — Opt-in layer limits: --no-tracks-on and --layer-cost

Decision

Layer limits are two opt-in router options for all nets: --no-tracks-on (no new tracks; vias and plane connections stay) and --layer-cost (integer per-mille factor on track length, never below 1)

Alternatives

Read the limit from KiCad's layer type (power); allow factors below 1 to prefer a layer; per-net limits on the command line

Reason

Issue #6. Layer types are often left at signal or set to power on layers that do carry signals, so the type would change results on boards whose owners asked for nothing. Below 1 the search heuristics stop being lower bounds. Per-net limits are KiCad custom rules. Six 4-layer boards: inner-layer track 34 % → 0 % (1,058 → 1,038 routed) or → 6 % at factor 4 (1,054 routed), no added errors (doc 05 §29)