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Creates a lazy, caller-owned projection of this source, supporting nullable values and chained projections. Construction does not subscribe or evaluate transform. Keep the returned identity when consumers share a projection. The transformation must be deterministic, side-effect-free, and return normally; ordinary subscriptions may transform on any notifying thread. Ordinary subscriptions preserve every revision, including equal results. Retained UI consumers instead share the upstream subscription, transform a frame-committed input, and suppress dependent UI work when the transformed result is equal by ==. No source history or global cache is retained. Heavy work and I/O belong outside the transformation.

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