perf: cache the artifact shell, minify its inner renderer, and prefetch it early

[fforres]

Why

Opening an artifact downloaded the MCP-Apps shell document again on every view: it is about 4.7 MB raw (about 950 KB gzip), and hashed assets were served with max-age=0, must-revalidate and no ETag. On a cold open the download also waited for the artifact row, because the iframe mounts only after the row arrives.

What changes

• host-cloudflare: a build-only plugin emits _headers. /assets/* (content-hashed) becomes public, max-age=31536000, immutable. The stable-named shell copy read by the MCP session keeps revalidating.
• mcp-apps-shell: bundleInnerRenderer minifies in production. The shell drops from 4,720,043 B to 2,733,014 B raw, and from 968,294 B to 755,385 B gzip. The smoke harness stays unminified.
• react: preloadArtifactRenderer adds one <link rel="prefetch"> for the shell document once the renderer chunk resolves. The shell module exports shellHtmlUrl, so the document download overlaps the row fetch. packages/react still does not import the shell package.

Tests

• artifact-renderer.test.tsx: one prefetch link per URL, and none when no URL is announced.
• vite.test.ts passes.
• Typecheck and oxlint are clean for the touched packages.

https://claude.ai/code/session_019ZE3j9YzRKxT4LaiDCBv6S