Ports the foundational board classes: Unit (mil/inch/mm/um with
micrometer scaling), Layer/LayerStructure (the layer stack with name and
signal-layer lookups), CoordinateTransform (DSN<->board scaling by the
resolution), Net/Nets (connectivity keyed by (name, subnet) and a board-
unique net number), and ClearanceMatrix (class x class spacing with a
reserved null class and a default class).
The per-layer axis of ClearanceMatrix is simplified to a single value per
class pair (DSN default clearance rules are layer-independent for the
boards we build); the router phase can add the layer dimension.
Unit-tested: unit scaling/parsing, layer lookups, transform round-trip,
net registration/lookup, and clearance default/append/symmetry.
Ryan Malloy
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