Function rfc3339_millis

pub fn rfc3339_millis(unix_ms: u64) -> String

Unix millis → YYYY-MM-DDTHH:MM:SS.sssZ (UTC), no date crate needed.

Device-emitted envelopes stamp their own occurred_at (the façades never pass one), and the server’s envelope schema requires an ISO-8601 datetime — so the formatter lives next to the clock. civil-date math is Howard Hinnant’s days-to-ymd; the output also satisfies api.rs’s is_plausible_iso8601 gate on the enqueue path.