Verify immutable browser navigation snapshots

[jt55401]

Summary:

• parse and freeze signed sections from the post-load same-URL response
• map source snapshots to live elements by signed identity
• invalidate and re-verify mutations and SPA navigations
• keep status indicators outside signed content
• add a top-down README and one-command Docker validation

Validation:

• 60 Jest tests
• TypeScript typecheck
• ESLint: 0 errors, 82 existing warnings
• Chromium, Firefox, and Safari production builds

All validation ran through scripts/test-in-docker.sh.