Add three test files designed to stress the control plane under
concurrent and adversarial conditions:
- race_test.go: 14 tests exercising concurrent mutations, session
replacement, batcher contention, NodeStore access, and map response
delivery during disconnect. All pass the Go race detector.
- poll_race_test.go: 8 tests targeting the poll.go grace period
interleaving. These confirm a logical TOCTOU race: when a node
disconnects and reconnects within the grace period, the old
session's deferred Disconnect() can overwrite the new session's
Connect(), leaving IsOnline=false despite an active poll session.
- stress_test.go: sustained churn, rapid mutations, rolling
replacement, data integrity checks under load, and verification
that rapid reconnects do not leak false-offline notifications.
Known failing tests (grace period TOCTOU race):
- server_state_online_after_reconnect_within_grace
- update_history_no_false_offline
- rapid_reconnect_peer_never_sees_offline
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