Fixes #1873

Summarize your change.

Part of ASWF Dev Days 2026. Removes redundant Marker::Color enum and replaces it with the standard Color class recently added for metadata.

• replace Marker::Color and its std::string _color attribute with an optional Color equivalent (defaults kept the same)
• updates Marker to schema 3
  • adds upgrade function: if the color name matches any of the previous Marker::Color enum names, it converts it to the Color equivalent. Otherwise, it preserves the original name
  • adds a downgrade function: if the Color name matches any of the Marker::Color enum names, it renames it to the all-caps name. Otherwise, it preserves the original name
• maps the MarkerColor Python name to use Color. We can mark the name for deprecation in the future, but I figured this would be helpful for backwards compatibility for now
• related issue, found when updating tests: adds != operator for Color class, binds the equality functionality into Python

Reference associated tests.

• Marker-related baseline files updated
• updated test_marker.py tests, added a test for upgrading schema to 3
• updated test_clip.cpp, test_clip.py, test_console.py that references marker colors