Optional ReadonlydatasetsDatasets to add at start-up. Optional — a host with a
SourceRegistry can instead load datasets on demand by identity,
which is the shape that avoids sending data to workers at all.
ReadonlyregistryOptional ReadonlysourcesOptional Readonlyunits
What a worker's setup module returns — the same shape Host takes, plus datasets to seed.
Named as a module specifier rather than passed as a value because a registry is functions, and functions do not survive structured clone. Both isolates import the same module; the ops are shared code.