A2uiSurfaces

The surfaces a transcript amounts to, and how to get a renderer from the last transcript to this one.

a2ui-core refuses to create a surface that exists, and upstream's middleware re-sends createSurface in every cumulative snapshot -- so a renderer fed the transcript's payloads as they came would throw on the second snapshot. This type sits between: it remembers what each carrier last had applied, and when a carrier's payload changes it deletes the surfaces that carrier created and replays the new payload whole. Delete-then-replay rather than a diff, because a snapshot is one document -- the specification's normative reading of replace -- and the cost is a redraw of one surface, which is what a new snapshot means anyway.

Immutable: accept returns the Step to the next value, and the caller that applied the step's messages keeps Step.next. A caller whose renderer rejected a batch keeps rejected instead, so this value never claims a surface the renderer does not hold.

The caller applies Step.deletes first and then each Step.batches entry as one atomic applyAll. A2uiRenderer.applyAll leaves its state untouched when a batch throws, which is why the batches are per carrier rather than one list: a payload one carrier got wrong costs that carrier's surfaces and nobody else's.