Function is_explicit_source_timestamp_compatibility 

pub(crate) fn is_explicit_source_timestamp_compatibility(
    output_schema: &[ColumnSchema],
    output_lineage: &[Option<usize>],
    sink_schema: &[ColumnSchema],
    source_schema: &SchemaRef,
) -> bool

Returns whether an existing sink uses the legacy explicit source-time-index layout.

This check is deliberately separate from resolve_sink_layout: ordinary auto-column resolution remains the first choice, and this compatibility path is only for a sink whose trailing column is a real, user-defined time index.