Vite calls Rollup buildStart in serve mode but emitFile() is unsupported there, plugins must guard or split build/serve paths

A Vite plugin that uses this.emitFile() inside buildStart to produce a static asset (e.g. /.well-known/) will trip the warning context method emitFile() is not supported in serve mode. This plugin is likely not vite-compatible whenever vite dev boots, and the asset is never written to disk (there is no dist in serve mode). The hook still runs in serve, but the Rollup PluginContext is a stub. The same plugin will also be invisible under vite preview because Vite's preview uses sirv to serve dist, and sirv's default dotfiles setting hides paths under .well-known/. Verified against Vite 8.0 in 2026-05.

Detect command in configResolved (set a flag from config.command === 'build') and early-return from buildStart in serve mode. Mirror the same content via a configureServer middleware so /.well-known/<file> works during dev, and add configurePreviewServer with the same middleware so vite preview exposes it past sirv's dotfile filter. The build path still uses emitFile so the file ships in dist for static hosting. Verify by curling the path under both vite dev and vite preview.