Harden CI and add GitHub Pages site

[godofecht]

This hardens the repository around the existing CMake/test architecture and adds a proper static project site.

Changes include:

• replace the hard-coded README build badge with the real Actions badge and fix the clone URL
• build and test under both GCC and Clang with Ninja and the project’s existing FetchContent dependencies
• preserve tag-driven release packaging while making CI manually dispatchable and concurrency-safe
• add a GitHub Pages deployment workflow and a dependency-free project landing page under site/
• document the distinction between the static Pages site and the native C++-backed interactive playground
• add contribution, security-reporting and pull-request guidance

The Pages workflow is ready to deploy after merge. If Pages has not previously been enabled for the repository, GitHub may require selecting GitHub Actions as the Pages source once in repository settings.