Add a public signature registry and refresh manifesto sections

[alexsoyes]

Summary

• Add a public-facing signature registry surface and update the signature section to present entries more clearly
• Refresh the cover, values, principles, terminal, footer, and layout styles for a more cohesive manifesto presentation
• Update supporting definition/spec components and tokens to match the new visual system

Testing

• Not run (not requested)
• Changes were validated through the existing app structure and section-level updates