Give Mermaid diagrams a Chopin theme on stock Mermaid layout

Stock Mermaid keeps the parser and layout, so a diagram lays out the same
here as on GitHub. Only the drawing is restyled, through themeCSS written
entirely in theme tokens, plus a measured post-render step for arrow gaps,
rounded class boxes and longer sequence lifelines.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>