registerStrip method
Called by SSpreadsheet when a strip mounts. isPrimary marks the
column-header strip, which outlives every body row.
Implementation
void registerStrip(ScrollController controller, {required bool isPrimary}) {
if (_disposed) return;
_strips[controller] = isPrimary;
_electOwner();
// Deferred: this runs from the strip's initState, i.e. during a build.
// Notifying listeners synchronously there would mark them dirty mid-build.
_schedulePublish();
}