Move Playground hosting to Vercel

[alixander]

Human

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AI

Move the public D2 Playground to Vercel for production and pull request previews. Both play.d2lang.com and playground.d2lang.com serve the static build; preserve the existing Plausible script and event proxies. Remove the AWS deployment workflow and script so future production updates have a single deployment path.

The build pins Yarn 1.22.22 and esbuild 0.16.3, checks out the public editor submodules, and emits ordinary assets in dist so Vercel can apply HTTP compression. Assets retain their existing URLs and revalidate on subsequent visits. Add the Vercel Open Source Program README badge and production/rollback instructions.

Validation: the exact branch head deployed successfully on Vercel with Node 24; browser checks rendered Dagre, ELK, and TALA diagrams, restored a shared URL, and downloaded a valid SVG. JavaScript and CSS return Brotli encoding, the editor WASM loads, and the Plausible script proxy responds successfully. Both custom domains have valid Vercel TLS certificates prepared before DNS cutover. GitHub ci and nofixups checks passed, and git diff --check passed.

The companion infrastructure change adopts both Vercel DNS records and disables only the dedicated Playground CloudFront distribution while retaining the versioned S3 origin and shared certificate for rollback.

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