Add browser-local WordPress signing

[jt55401]

Summary

• create a non-extractable Ed25519 key in the author browser and retain it in IndexedDB
• prepare canonical v1 payloads in WordPress, verify browser signatures in PHP, and persist immutable signed output
• bind each local key ID to one SPKI document and protect completion tokens against replay
• queue scheduled and headless publications until the author signs from the editor
• document standalone installation, configuration, test, and browser-local signing flows

Validation

• ./wordpress/bin/test-docker.sh: 81 tests, 329 assertions
• Composer strict validation
• PHP syntax checks and JavaScript syntax check
• live WordPress editor signing in the frozen E2E scenario
• emitted section verified by the browser client through the public WordPress key endpoint

Closes #4