Support lookup against external tables #37

The executor is pure and synchronous and reads nothing but the document and the crosswalk. A lookup would need data from outside both, and an inverse only exists when the table is injective.

Decision: decline. An injected table could keep encode and decode deterministic for one call, but the crosswalk document would stop being the whole definition of both directions: decode inverts encode only if the caller supplies the identical table on both sides, and nothing in the document can check that. A crosswalk fetched by URL (#48) would then mean different things against different tables. An injective table already has a sound home in the format: a TypeScript caller can build the rule as { op: "enum", map: table, mapBack: invert(table) }, and parseCrosswalk checks that mapBack inverts map, so the bijection is validated and travels inside the document. A lookup node inside a when keeps throwing for the same reason.

Revisit if a table must stay outside the document for size, with a content hash pinned in the crosswalk so encode and decode can refuse a table that differs from the one the crosswalk was written against.

The README reason for this omission is updated in #56.