Docs: add a codebase metrics page (cloc)

Adds a "Codebase metrics" page to the documentation (mkdocs): a measured snapshot of the codebase and all submodules, counted with cloc.

It shows lines by language, a production vs test split, the code/comment/blank composition, and a per-submodule ranking, rendered as static inline SVG charts plus Markdown tables. No JavaScript or charting plugin is needed, so it builds in the docs as-is. A "Codebase metrics" nav entry is added to mkdocs.yml. Refreshed periodically.