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

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Repositories/AuditLogRepository.cs

@@ -23,7 +23,8 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Repositories;
 public class AuditLogRepository : IAuditLogRepository
 {
     // SQL Server error numbers for duplicate-key violations on the
-    // partition-aligned clustered PK_AuditLog (EventId, OccurredAtUtc).
+    // partition-aligned clustered PK_AuditLog (OccurredAtUtc, EventId) or the unique
+    // UX_AuditLog_EventId_Occurred (EventId, OccurredAtUtc) — ScadaBridge#111.
     // 2601 is a unique-index violation; 2627 is a primary-key/unique-constraint
     // violation. The IF NOT EXISTS … INSERT pattern has a check-then-act race
     // window — two sessions can both pass the EXISTS check and then both attempt
@@ -504,8 +505,9 @@ VALUES
         // guarded and the staging table is GUID-suffixed, so a re-run is idempotent.
         //
         // ALIGNED UNIQUENESS: there is no longer an index drop/rebuild around
-        // the SWITCH. EventId uniqueness is now enforced solely by the clustered
-        // PK_AuditLog (EventId, OccurredAtUtc), which is partition-aligned on
+        // the SWITCH. EventId uniqueness is enforced by the clustered PK_AuditLog
+        // (OccurredAtUtc, EventId since FlipAuditLogClusteredKey, ScadaBridge#111) and the
+        // unique UX_AuditLog_EventId_Occurred, both partition-aligned on
         // ps_AuditLog_Month(OccurredAtUtc) — so ALTER TABLE … SWITCH PARTITION has no
         // non-aligned index to object to. The former dance dropped UX_AuditLog_EventId,
         // switched, then rebuilt it OFFLINE inside the same transaction: a whole-table
@@ -595,10 +597,21 @@ VALUES
                 ParentExecutionId  AS CAST(JSON_VALUE(DetailsJson,'$.parentExecutionId') AS uniqueidentifier) PERSISTED,
                 IngestedAtUtc      AS CAST(SWITCHOFFSET(CAST(JSON_VALUE(DetailsJson,'$.ingestedAtUtc') AS datetimeoffset), 0) AS datetime2(7)),
                 ChainSeq           bigint           NULL,
-                RowHash            binary(32)       NULL,
-                CONSTRAINT PK_{stagingTableName} PRIMARY KEY CLUSTERED (EventId, OccurredAtUtc)
+                RowHash            binary(32)       NULL
             ) ON [PRIMARY];
 
+            -- 2b. The staging clustered key must match dbo.AuditLog's or SWITCH refuses
+            --     (msg 4913/4947). FlipAuditLogClusteredKey (ScadaBridge#111) changed it from
+            --     (EventId, OccurredAtUtc) to (OccurredAtUtc, EventId), so read the live key
+            --     order instead of assuming one: this binary purges either shape, before or
+            --     after that migration (and after its Down).
+            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'OccurredAtUtc'
+                ALTER TABLE dbo.[{stagingTableName}] ADD CONSTRAINT PK_{stagingTableName} PRIMARY KEY CLUSTERED (OccurredAtUtc, EventId);
+            ELSE
+                ALTER TABLE dbo.[{stagingTableName}] ADD CONSTRAINT PK_{stagingTableName} PRIMARY KEY CLUSTERED (EventId, OccurredAtUtc);
+
             -- 3. Switch the partition out. $partition.pf_AuditLog_Month returns
             --    the partition number that contains the supplied boundary value;
             --    SWITCH PARTITION N moves that partition's pages to the staging