New Puzzle: Monochrome Chess

Every move has to be a capture of the opposite colour, colours don't have to alternate. Goal is to eliminate one of the colours.

Interesting problems should involve capturing both colours.

No files, tests, or entry points are named. Start by locating how existing chess variants and puzzles are represented and tested, then determine where the monochrome-capture rules and elimination goal belong. Done means the new puzzle is available with its stated rules and coverage for capturing both colours.

Written by the indexing model from the issue text.