refactor(css): inline FL-Builder layout grid + add visual regression gates

[pftg]

Summary by CodeRabbit

• Styling

  • Added a layout-grid include to the critical header and consolidated per-page critical styles; homepage critical CSS expanded into a modular stylesheet with responsive rules for tabs, menus, logos, headers and many builder modules; contact/privacy pages now use the shared consolidated critical bundle.

• Documentation

  • Added extensive CSS migration program docs (index, goals, tracker, dashboard, strategies, runbooks) and removed/archived numerous legacy planning and sprint artifacts.

• Tests

  • Introduced strict zero-tolerance visual‑regression and CSS processing validation protocols and associated tester/reviewer workflows.