Module Granary_ivm.DeltaSource

Lift row-mutation events into Z-set deltas (#417 Phase 3).

This is the bridge from a change feed (e.g. the engine's row-level delta feed, #419) to the incremental operators: an inserted element is +1, a deleted element is -1, and an update is the retraction of the old element plus the insertion of the new (-old +new) — so an update to an identical element is a no-op, and a sequence of events folds to a single Z-set delta.