Yields params.data from events of a single method.
Covers the channels whose projection is just "emit the payload": values,
updates, checkpoints, tasks — the SDK analog of local's
Values/Updates/Checkpoints/TasksTransformer, all of which push
params["data"] unchanged. The REST-state seeding for values stays at
the projection layer; it is a one-shot pre-stream fetch, not part of the
event state machine.
The protocol method this decoder consumes.
When not None, events whose namespace differs are ignored
(scope filter, mirroring the local transformers' namespace != scope
check). None consumes every namespace — the historical values
projection behavior, where subscription scoping is handled upstream.
