serve static assets from the build manifest #16565

Describe the problem

The static file server re-derives at request time what the build already computed, and each gap surfaces as its own bug:

• MIME types: sirv bundles its own mrmime, so kit's utils/mime.js additions never reach adapter-node (No Content-Type when serving .ico files #13753; patched at the response layer by fix: serve static files with the Content-Type recorded in the manifest #16564)
• Vary: Accept-Encoding sent for assets that were never precompressed (fix: don't send "Vary: Accept-Encoding" header when precompress=false #15182, fix: don't send Vary: Accept-Encoding for assets that were never precompressed #16566)
• filenames ending in + 404 in production but resolve in dev (adapter-node does not serve filenames that end with + #11766, still reproduces on 3.0.2)
• startup totalist walk rebuilds a lookup the build already has; ETag is mtime-derived, not content-derived

Describe the proposed solution

Record the two things adapt time knows and currently discards (which files builder.compress produced variants for, and a content hash), then serve from the manifest: lookup and stream. Type from manifest.mimeTypes, Vary only where a variant exists, exact keys instead of path-guessing, no startup walk. sirv, totalist, @polka/url and mrmime drop out of adapter-node as a consequence. Exporting the server from @sveltejs/kit/node keeps adapter-node on one copy.

Alternatives considered

Upstreaming: sirv#173 open unreviewed since January, last release September 2025. Vendoring sirv's 249 lines as-is: fixes MIME but keeps the runtime re-derivation.

Importance

nice to have