Replaces M1's NotSupportedException stub with the production drop-DROP-INDEX
→ CREATE-staging → SWITCH PARTITION → DROP-staging → CREATE-INDEX dance
documented in alog.md §4. UX_AuditLog_EventId is intentionally non-aligned
with ps_AuditLog_Month so single-column EventId uniqueness can be enforced
cheaply for InsertIfNotExistsAsync; SQL Server rejects ALTER TABLE SWITCH
while a non-aligned unique index is present, so the implementation drops
it, switches the partition data into a GUID-suffixed staging table on
[PRIMARY], drops staging (discarding the rows), and rebuilds the unique
index — all inside an explicit transaction with a CATCH that guarantees
the unique index is rebuilt regardless of failure point.
Also adds GetPartitionBoundariesOlderThanAsync to IAuditLogRepository: a
CROSS APPLY over sys.partition_range_values + per-partition MAX(OccurredAtUtc)
to enumerate retention-eligible months for the M6 purge actor (next commit).
Tests verify:
* Old partition's rows are removed; other months untouched
* UX_AuditLog_EventId is rebuilt after a successful switch
* InsertIfNotExistsAsync's first-write-wins idempotency still holds after switch
* On engineered SWITCH failure (inbound FK from a probe table), SqlException
propagates AND UX_AuditLog_EventId is still present (CATCH branch ran)
* GetPartitionBoundariesOlderThanAsync returns only boundaries whose partition's
MAX(OccurredAtUtc) is strictly older than the threshold; empty partitions
excluded
M4 Bundle B (B1) — add the central-only ICentralAuditWriter implementation
and inject it into NotificationOutboxActor so subsequent tasks (B2/B3) can
route attempt + terminal lifecycle events through the direct-write audit path.
- CentralAuditWriter: thin wrapper around IAuditLogRepository.InsertIfNotExistsAsync;
scope-per-call (matches AuditLogIngestActor / NotificationOutboxActor pattern);
stamps IngestedAtUtc; swallows all internal failures (alog.md §13).
- Registered as a singleton in AddAuditLog.
- NotificationOutboxActor ctor takes ICentralAuditWriter (validated non-null).
- Host wiring resolves the writer once from the root provider and passes it
into the singleton's Props.Create call.
- Existing TestKit fixtures updated with a NoOpCentralAuditWriter helper so
tests that don't exercise audit emission still compile and pass.
