chore: harden SEO and frontend metadata checks

[CoderDeltaLAN]

Hardens public metadata and repository verification.

Scope:

• add canonical, robots, author and Open Graph metadata;
• switch Twitter card to summary because there is no final social image yet;
• remove misleading http-equiv security metadata that GitHub Pages does not guarantee as real headers;
• tighten CSP script-src;
• add verify.sh checks for SEO/security metadata.

Constraints:

• README.md untouched;
• no backend;
• no secrets;
• no generated/vendor artifacts.

Local validation:

• scripts/verify.sh OK;
• git diff --check OK;
• pre-push validation OK.