Function micros_to_iso 

pub fn micros_to_iso(micros: i64) -> String

Convert a microsecond epoch i64 to an RFC 3339 / ISO-8601 string.

Entity and Note storage uses i64 microseconds internally; this is the single conversion point before any field reaches the MCP boundary.

Format: YYYY-MM-DDTHH:MM:SS.ffffffZ (SecondsFormat::Micros, UTC Z).