GetDataReadingBuilderScope

Registers builder as a child of this scope, snapshotting the CompositionLocal values currently active (via BuildContext.get) into GenericBuilder.compositionLocals — captured now, at DSL-call time, not later when build actually invokes it. Called once per nested DSL call (SimpleText(...), SendNetworkRequest(...), etc.) by each such call's own top-level builder function.

Builds every registered Builder in registration order, restoring each one's own GenericBuilder.compositionLocals snapshot (via BuildContext.with) for the duration of its own GenericBuilder.build call.

Declares one value to read inside a GetData event.