feat: full MusicXML key-mode support in mx::api

[rpatters1]

Human Summary

Extend API KeyMode support to include all known values on both read and write. Unknown values are handled as unsupported.

Summary

KeyMode in KeyData.h only had major and minor. MeasureReader mapped every other
<mode> to KeyMode::unsupported and PropertiesWriter wrote a <mode> element only for
major and minor, so a keyless signature (<fifths>0</fifths><mode>none</mode>) could be
neither read nor written.

KeyMode now covers the ten standard MusicXML modes. Reading and writing both go through a
new bridge table in Converter instead of a hand-rolled if-chain, which retires the
// TODO - support all modes in MeasureReader. The new enumerators are appended, so the
existing values are unchanged.

Distinctions that are preserved:

• unspecified writes no <mode> element; none writes <mode>none</mode>
• zero fifths implies no mode, so C major, A minor, none, and unspecified stay four
  distinct states
• unsupported is kept, and still earns its keep: <mode> is an open vocabulary, so it is
  what an unrecognized value reads back as. It writes nothing.
• none is an ordinary mode value. It writes a traditional key and is not coupled to
  nontraditional key handling.

No print-object support, and nothing Finale-specific: this is general mode support.

lysuite/ly13b_KeySignatures_ChurchModes.xml is pinned to the api roundtrip to defend the
feature. On the base commit it fails with child count mismatch at the key element (the
dropped <mode>), and passes with this change.

Testing

• ly13b_KeySignatures_ChurchModes.xml fails before the change, passes after
• 8 new KeyData tests: every value serializes to its exact spelling, every standard mode
  deserializes, zero-fifths none round-trips, absent <mode> stays unspecified, an
  unrecognized mode reads as unsupported, and the four zero-fifths
• All KeyData tests pass (*_KeyData: 285 assertions in 20 test cases)
• make api-test passes (5366 assertions in 471 test cases)
• make api-roundtrip passes (296 of 296 pinned)
• make core-roundtrip-test passes (837 test cases)
• make core-unit passes (212 assertions in 41 test cases)