Function encode_props 

pub fn encode_props(props: Option<&Map<String, Value>>) -> String

Serialize a props map into the data-devup-props marker payload.

Returns an empty string when there is nothing worth marking (all keys either skipped, null, or the map was None). The caller is expected to omit the attribute entirely in that case.

Payload: base64(utf8_bytes(json_stringify(filtered_props))) using standard base64. Attribute-safe by construction.