Add Cloudflare static artifact import proof

[chubes4]

Summary

• replace the placeholder artifact with a self-explanatory WordPress Playground on Cloudflare Workers site
• import the two-page site through pinned Static Site Importer v1.3.6 with zero fallback, HTML, freeform, or invalid blocks
• require exact source-to-WordPress visual parity before editing and republishing native block content
• preserve structured SSI failure details and verify post-restart block edit persistence

Closes #1975.

Immutable dependencies

• php-transformer v0.4.10: https://github.com/Automattic/blocks-engine/releases/tag/php-transformer-v0.4.10
• Static Site Importer v1.3.6: https://github.com/Automattic/static-site-importer/releases/tag/v1.3.6
• SSI archive SHA-256: fb3679d679c03422fa77f5f33bc549fa869b0ce2bb16f7431705af4fc16b97db

Verification

• npm run test:cloudflare-runtime
• npm run cloudflare:local-gate:d1
• npm run cloudflare:local-gate:provisioning
• /: 0 mismatched pixels at 1280x720
• /about/: 0 mismatched pixels at 1280x720
• import receipt: 0 fallback, core/html, freeform, and invalid blocks
• authenticated native-block edit, automatic republication, cold restart, and persisted raw block content passed

Upstream fixes

• synthetic paragraph margins: Reset margins on synthetic paragraph wrappers blocks-engine#717
• label/value row geometry: Preserve label/value row geometry blocks-engine#723
• SSI dependency update: Update PHP transformer to 0.4.10 static-site-importer#755

AI assistance

OpenCode with OpenAI GPT-5.6 Sol helped implement the fixture and gate, diagnose upstream conversion defects, update immutable release pins, and run the verification workflows. Chris Huber remains responsible for every line.