TypeMapper

A bijection between a proto type and the Scala type a field should actually have.

This is the single extension point behind both user-declared (nscn.polaris2.protoc.field).type options and the built-in Well-Known Type mapping: the generated field has type Custom, while the wire format and reflective access stay in terms of Base.

toBase(toCustom(b)) == b is expected to hold. Where it cannot - java.time.Instant normalizes, so a Timestamp with nanos >= 1e9 does not come back unchanged - the mapping is documented and can be turned off per field.