Function capture

Run fn against store as a single recorded transaction: store is mutated in place, and the resulting change-set is returned as a TransactionResult.

The returned result is a replicable delta: its redo operations can be forwarded to a peer and replayed there with applyOperations, so a pluggable replication strategy can propagate an out-of-transaction store edit — e.g. a version-upgrade migration run inside a load handler — through whatever transport it chooses. Core stays propagation-agnostic; this only captures.

The transactional wrapper is created and discarded internally, so this is the "wrap the store, do the work as one transaction, keep the change, throw the wrapper away" pattern in a single call. Ops inside fn must go through the provided t (not the outer store) to be recorded.