Function format_instant 

pub fn format_instant(dt: DateTime<Utc>, format: BindFormat) -> String

Format an already-resolved UTC instant per BindFormat. Unlike format_bookmark (which takes an arbitrary scalar and, for BindFormat::Raw, echoes it verbatim), this always has a real instant, so Raw and Iso8601 both emit an RFC 3339 UTC timestamp. Used to render window boundaries (#527).