Function createRebaseReplayConcurrency

Concurrency strategy that implements the optimistic rebase-replay protocol required for multi-peer synchronisation.

Behaviour:

• Local "commit" intents are applied as transients (negative time) and wait for the sync server's echoed committed envelope to promote them via rollback-and-replay. This is required for cross-peer entity-id determinism under concurrent edits.
• Every outbound envelope is stamped with userId so the reconciler's compound (userId, id) key keeps concurrent peers' id counters from colliding.
• onBeforeToData / onAfterToData roll back and replay the transient queue around serialisation so snapshots always contain only committed state.