Wait for artifact layouts before rendering previews

[an1va]

What changed

• Sample visible DOM geometry after network idle in both preview renderers.
• Require three identical, ready layout samples before storing a screenshot.
• Keep waiting while eager images are incomplete.
• Fail the render when the layout never settles instead of saving a convincing blank shell.

Why

Client-rendered artifacts can add their primary gallery after network idle. The preview worker previously captured that transition and stored a shell with an empty central canvas. This makes render readiness depend on stable visible layout rather than network timing alone.

Verification

• pnpm verify
• 250 API test files passed; 2 skipped
• 2,380 API tests passed; 3 skipped
• Node and Cloudflare Worker typechecks passed
• Added focused coverage for late gallery population, pending images, and never-settling layouts

Reproduction artifact

https://derive.to/artifacts/sift-for-accenture-recommended-single-and-packet-2ks00lnp

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[github-actions]

Preview

https://derive-pr-755.derive-to.workers.dev

Deployed from af0734c as derive-pr-755.

It shares production's database — sign in with your real account, and treat anything you change here as changed for real. It has no routes, no cron, no queue consumer and no OG renderer, so it cannot serve derive.to, run scheduled work, or write images onto real artifacts.

Unlike production it serves artifact HTML on its own origin (that is what makes frame-side changes visible here). Storage is still sandboxed away, but untrusted HTML and the sign-in form share a hostname — treat this URL as you would any link: don't type a password into it because a page asked you to.