Prove persisted Gutenberg edits in solved fixture gate

[chubes4]

Summary

• require solved-candidate runs to insert fixture-specific content through Gutenberg, save, reload, and verify the marker in persisted front-page post_content
• run wp.blocks.validateBlock against the post-save document and fail closed on action, persistence, reload, or validation errors
• keep ordinary development matrix runs unchanged and document the stronger solved-fixture contract

Verification

• npm run test:fixture-matrix (259 tests passed)
• direct committed solved-only WP Codebox bench: 2 fixtures passed, 0 failed, 0 findings
• 15-saas: persistence confirmed; 288/288 post-save blocks valid
• 89-static-site-importer-architecture: persistence confirmed; 171/171 post-save blocks valid

AI Assistance

OpenAI GPT-5.6 Sol via OpenCode inspected the fixture gate, implemented the edit/save/reload persistence proof, added tests and documentation, and ran the runtime verification. Chris Huber reviewed the work and remains responsible for every line.