Check local write fairness at receiver flush boundaries - #12
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Problem
Observing how much replication backlog remains when a local caller wakes up makes fairness assertions depend on scheduler timing. A local write can also complete eventually while still waiting behind the entire finite backlog, masking a missing fairness turn.
Fix
Generate receiver buffer sizes, incoming batch sizes, backlog lengths, and local register/join operations for both registry and PG receiver lanes.
Suspend one shard, enqueue the receiver backlog, and explicitly wait until the public local request is queued before resuming. Assert that the local event appears at the first receiver flush and before the backlog finishes. Check the remaining remote event order, the exact local event, settled contents, and index consistency.
Supporting information
Stacked on #11 in the follow-up chain rooted at #8. This PR adds only fairness properties and documentation, reusing the existing fixture and history oracle.
The property controls a finite backlog on one shard. It does not claim a deterministic BEAM scheduler or fairness under an infinite stream of control traffic. Production behavior is unchanged.