feat(audit): flip AuditLog clustered key to (OccurredAtUtc, EventId) (ScadaBridge#111)

Variant B2 of the #78 benchmark: migration FlipAuditLogClusteredKey drops the
twelve non-clustered indexes, re-adds PK_AuditLog as (OccurredAtUtc, EventId)
on ps_AuditLog_Month, adds the aligned unique UX_AuditLog_EventId_Occurred
(EventId, OccurredAtUtc) for the idempotency probe, and rebuilds the twelve
indexes unchanged. Guarded (re-runnable) Up, inverse-rebuild Down; production
scripts docs/plans/sql/FlipAuditLogClusteredKey{,.rollback}.sql.

The partition-switch purge staging table now reads the live clustered key
order from sys.index_columns, so it switches either shape (old binaries
hard-code the old order: ship with the binaries, both centrals stopped).

Evidence: 300k rows, page 1 90,532 -> 34 logical reads (ordered backward
clustered scan under Top), keyset page 90,532 -> 46, probe 14 -> 14;
FlipAuditLogClusteredKeyMigrationTests pins indexes, grants, Down/Up, the
staging key (mutation: hard-coded old key fails SWITCH) and the read ratio.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

docs/plans/sql/FlipAuditLogClusteredKey.sql

@@ -0,0 +1,63 @@
+-- FlipAuditLogClusteredKey (2026-11-10, ScadaBridge#111, variant B2 of
+-- docs/plans/2026-11-09-auditlog-clustered-key-benchmark.md). dbo.AuditLog clustered key
+-- (EventId, OccurredAtUtc) -> (OccurredAtUtc, EventId) + aligned unique UX_AuditLog_EventId_Occurred
+-- (EventId, OccurredAtUtc); the twelve non-clustered indexes are dropped first and rebuilt last,
+-- unchanged and partition-aligned. Re-runnable (no-op on an already-flipped table).
+--
+-- OFFLINE whole-table rebuild: size the window from the real table first
+--   SELECT SUM(used_page_count)*8/1024 AS MB FROM sys.dm_db_partition_stats WHERE object_id = OBJECT_ID('dbo.AuditLog');
+-- (local reference: 631 MB / 300k rows rebuilt in ~9 s on an idle dev container; production I/O and
+-- log growth differ — the whole table is rewritten in one transaction, so the log needs ~2x the
+-- table size free). Take a backup first. Stop BOTH central nodes (old binaries' partition-switch
+-- purge cannot switch a flipped table: their staging table hard-codes the old key — msg 4947,
+-- rolled back, nothing purged). Rollback: FlipAuditLogClusteredKey.rollback.sql (same cost).
+--
+-- Run with QUOTED_IDENTIFIER ON (sqlcmd -I, or keep this header): AuditLog has persisted computed
+-- columns and filtered indexes and SQL Server raises error 1934 otherwise.
+SET QUOTED_IDENTIFIER ON;
+SET ANSI_NULLS ON;
+GO
+BEGIN TRANSACTION;
+
+IF 
+(SELECT c.name FROM sys.index_columns ic
+   JOIN sys.columns c ON c.object_id = ic.object_id AND c.column_id = ic.column_id
+  WHERE ic.object_id = OBJECT_ID('dbo.AuditLog') AND ic.index_id = 1 AND ic.key_ordinal = 1) = N'EventId'
+BEGIN
+
+DECLARE @dropSql nvarchar(max) = N'';
+SELECT @dropSql = @dropSql + N'DROP INDEX ' + QUOTENAME(i.name) + N' ON dbo.AuditLog;' + CHAR(10)
+FROM sys.indexes i
+WHERE i.object_id = OBJECT_ID('dbo.AuditLog') AND i.type = 2 AND i.name IN (
+    N'IX_AuditLog_OccurredAtUtc', N'IX_AuditLog_Site_Occurred', N'IX_AuditLog_CorrelationId',
+    N'IX_AuditLog_Channel_Status_Occurred', N'IX_AuditLog_Target_Occurred', N'IX_AuditLog_Execution',
+    N'IX_AuditLog_ParentExecution', N'IX_AuditLog_Node_Occurred', N'IX_AuditLog_Kind_Occurred',
+    N'IX_AuditLog_Actor_Occurred', N'IX_AuditLog_Unchained', N'IX_AuditLog_ChainSeq',
+    N'UX_AuditLog_EventId_Occurred');
+EXEC sp_executesql @dropSql;
+    ALTER TABLE dbo.AuditLog DROP CONSTRAINT PK_AuditLog;
+    ALTER TABLE dbo.AuditLog ADD CONSTRAINT PK_AuditLog PRIMARY KEY CLUSTERED (OccurredAtUtc, EventId)
+        ON ps_AuditLog_Month(OccurredAtUtc);
+    CREATE UNIQUE NONCLUSTERED INDEX UX_AuditLog_EventId_Occurred ON dbo.AuditLog (EventId, OccurredAtUtc)
+        ON ps_AuditLog_Month(OccurredAtUtc);
+
+CREATE NONCLUSTERED INDEX IX_AuditLog_OccurredAtUtc ON dbo.AuditLog (OccurredAtUtc DESC) INCLUDE (Status) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Site_Occurred ON dbo.AuditLog (SourceSiteId ASC, OccurredAtUtc DESC) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_CorrelationId ON dbo.AuditLog (CorrelationId) WHERE CorrelationId IS NOT NULL ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Channel_Status_Occurred ON dbo.AuditLog (Category ASC, Status ASC, OccurredAtUtc DESC) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Target_Occurred ON dbo.AuditLog (Target ASC, OccurredAtUtc DESC) WHERE Target IS NOT NULL ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Execution ON dbo.AuditLog (ExecutionId) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_ParentExecution ON dbo.AuditLog (ParentExecutionId) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Node_Occurred ON dbo.AuditLog (SourceNode, OccurredAtUtc) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Kind_Occurred ON dbo.AuditLog (Kind ASC, OccurredAtUtc DESC) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Actor_Occurred ON dbo.AuditLog (Actor ASC, OccurredAtUtc DESC) ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_Unchained ON dbo.AuditLog (OccurredAtUtc, EventId) WHERE ChainSeq IS NULL ON ps_AuditLog_Month(OccurredAtUtc);
+CREATE NONCLUSTERED INDEX IX_AuditLog_ChainSeq ON dbo.AuditLog (ChainSeq) WHERE ChainSeq IS NOT NULL ON ps_AuditLog_Month(OccurredAtUtc);
+END
+
+INSERT INTO [__EFMigrationsHistory] ([MigrationId], [ProductVersion])
+VALUES (N'20261110033732_FlipAuditLogClusteredKey', N'10.0.13');
+
+COMMIT;
+GO
+