The engine call contract

Two rules decide almost every API question in this package. Both were arrived at by getting them wrong first.

A call answers, or it throws where it was written

An engine call is synchronous and takes no resource that could be missing: a font is required at construction, text and spans are validated there, and constraints are checked by the call itself. Nothing is left to wait for and nothing is left for a caller to get wrong, so there is no failure to hand back.