bench: keep legacy PK arm truly variable-length (AutoFixedWidthRecords opt-out)

DatabaseConfig.AutoFixedWidthRecords (default true, landed in perf/fixedwidth-default) makes new PK tables fixed-width; the --pk legacy arm now opts out so the comparison keeps measuring the legacy layout against the fixed-width arm.

Author: MPCoreDeveloper

tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs

@@ -353,6 +353,10 @@ static DatabaseConfig BuildConfig(SharpCoreDB.Interfaces.StorageEngineType engin
         {
             NoEncryptMode = true,
             StorageEngineType = engineType,
+            // The fair PK comparison intentionally isolates the record-layout variable: the legacy
+            // arm opts out of the AutoFixedWidthRecords default so it measures true variable-length
+            // records; the fixed-width arm forces FixedWidthRecordLayout.
+            AutoFixedWidthRecords = !fixedWidth,
             FixedWidthRecordLayout = fixedWidth,
             UseGroupCommitWal = false,
             EnableAdaptiveWalBatching = false,