No MEI exporter: MEI is import-only

[alessonqueirozdev-hub]

Problem

MEI import exists and covers CMN reasonably well. There is no MEI exporter — parseMEI / parseMeiScore have no counterpart, so MEI is a one-way door.

To be clear about the promise: the README has never claimed MEI round-trip, and the compatibility table marks import/render honestly. This issue exists so the gap is tracked rather than merely not-promised.

Why it is worth doing

MusicXML export exists and round-trips (including <notations><tuplet> and <time-modification>, fixed in 2.8.0). MEI is the format the musicology and digital-edition world actually archives in, and it is the format this package's data model is closest to — several MEI concepts are modelled natively here that MusicXML cannot express.

Scope

• <staffDef> with @clef.*, @key.*, @meter.*, @trans.semi / @trans.diat, @clef.dis / @clef.dis.place — all of which the importer already reads, so the mapping exists in one direction already.
• <layer> / <beam> / <tuplet> / <chord> / <rest> / <note> with @oct, @pname, @accid vs @accid.ges.
• <staffGrp> with @symbol and labels.

Acceptance criteria

• Round-trip tests in the same shape as the MusicXML ones: parse → export → parse, asserting the model is equal, not that the XML is byte-identical.
• Advanced modules that are model-only (see Advanced MEI modules are model-only: figured bass, mensural, <neume> import #32) are either exported or explicitly refused with a warning — not dropped in silence.