M6 head records M5 realities:
- IOptionsMonitor hot-reload pattern verified; M6 retention config can
reuse.
- AuditRedactionFailure counter site-only in M5; M6 wires central side.
- Filter integration is at 3 writer entry points; purge actor doesn't
emit so no filter integration needed.
- SwitchOutPartitionAsync drop-and-rebuild dance required (M1 reality
+ M6-T4 already documents it).
- M6 should land the real ISiteStreamAuditClient (Option A) so push
telemetry leaves NoOp behind.
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