refactor: enforce hex color resources

[han4wluc]

Summary

• make resources.colors[*].hex strict opaque hex only and enforce that during render-state resolution
• migrate VT specs and docs away from named/rgba color resources, moving transparency to element opacity where needed
• add project-data schema coverage for valid input, non-hex color rejection, dead projectData.contexts, and VT YAML parse health

Verification

• bun run test
• bun run vt:generate
• VT_DOCKER_TIMEOUT=180000 bun run vt:report