mx::core Support a newer version of MusicXML

[webern]

We are stuck on MusicXML 3.0 because it’s a big job to upgrade to 3.1 (or whichever version is out when we get to it).

• Reverse engineer MusicXML 3.0 codegen using AI
  • docs: add project skill #148
  • cleanup and gen project dir #149
  • gen: reverse engineer code generator for mx/core #152
• Improve AI's ability to innovate by creating core roundtrip tests that are not sensitive to symbols in the generated code.
  • core-dev mode for mx core roundtrips #153
  • gen: fix failing core roundtrip tests #155
• poject gen: expand core-dev test coverage #156
• Deshittify the 12,000 lines of reverse engineered python code in gen/ #157
• Generate MusicXML 4.1 code #158
• Analyze and surface differences to the mx/api layer. #159