Discriminant — lets snapshot consumers detect marker entries.
OptionalpreviewFirst SUMMARY_PREVIEW_LENGTH chars — primitives and strings only.
OptionalsizeSize proxy: string length, array length, or object key count.
typeof result, refined to 'array' / 'null' for objects.
Marker recorded in
StageSnapshot.stageWrites(and the commit observer's mutations payload) underwriteTracking: 'summary'(#13c-A). Same fields and cost contract as ReadSummaryMarker; distinct brand so consumers can tell which dial produced an entry.