Function resolve_generic_doc_markers 

fn resolve_generic_doc_markers(content: String) -> String

Scan content for .__MX_GENERIC_DOC__(...) marker lines, group by generic name, replace the first marker per generic with a synthesised standalone doc block, and delete all remaining markers.

This is the write-time counterpart to the proc-macro marker emission in s7_class.rs and s4_class.rs.