ChordSymbol.placement not exported/imported to/from MusicXML (<harmony placement="...">)

[yuanyelele]

music21 version

9.9.1

Operating System(s) checked
Windows 11

Problem summary
When setting ChordSymbol.placement (e.g. cs.placement = 'below') the value is not written into the MusicXML element as a placement attribute on export, and MusicXML is not read back into ChordSymbol.placement on import. The code contains # TODO: attrGroup: placement comments in both the exporter and parser, indicating this feature is intended but unimplemented.

This makes it impossible to preserve staff placement hints for chord symbols when round-tripping MusicXML.

Steps to reproduce

from music21 import stream, harmony
p = stream.Part()
cs = harmony.ChordSymbol('C')
cs.placement = 'below'   # expectation: saved to musicxml
p.append(cs)
p.write('musicxml', fp='tmp.xml')

Output:

      <harmony>
        <root>
          <root-step>C</root-step>
        </root>
        <kind>major</kind>
      </harmony>

Expected vs. actual behavior
If ChordSymbol.placement is set to 'above' or 'below', the MusicXML exporter should write it to .

More information
music21/harmony.py — the object (Harmony / ChordSymbol) needs a placement attribute (or to expose it if already present).
music21/musicxml/m21ToXml.py — exporter function chordSymbolToXml (TODO: attrGroup: placement).
music21/musicxml/xmlToM21.py — parser function xmlToChordSymbol / code path that constructs ChordSymbol from (TODO: attrGroup: placement).
music21/musicxml/helpers.py — contains useful helpers such as setXMLAttributeFromAttribute and setPlacement that can/should be reused.