feat: fail the build for dynamic fs-routes without generateStaticParams

[uhyo]

Summary

A dynamic route (e.g. blog/[slug]/page.tsx) without a generateStaticParams() export now fails the build instead of being skipped with a warning.

Motivation

The previous warning claimed the route "still works on the client via the SPA fallback", but that claim was only half-true:

• RSC payloads are serialized at build time with each entry's concrete params, so a server-component page reached via the SPA fallback renders with empty params — only "use client" pages reading useRouteParams() would get correct values.
• The SPA fallback itself only exists in the dev/preview servers; on real static hosting it requires host-level rewrite configuration the framework doesn't provide.
• Even where it "works", the visitor gets the index page's title/meta and a soft-200 instead of a 404 for invalid URLs.

Since a static site can only serve pages enumerated at build time, failing loudly is more honest than silently producing a route that mostly doesn't work.

Changes

• collectStaticPaths() throws a descriptive error for a dynamic route without generateStaticParams(); its now-unused onWarn parameter is removed (fs-routes is @experimental and exempt from semver).
• FsRouteModule.generateStaticParams JSDoc documents the export as required for dynamic routes.
• Unit test updated from "warns and skips" to "throws".
• Docs (FileSystemRouting.mdx) document the build failure and drop the SPA-fallback claim.

Note

Technically breaking for anyone relying on the old skip behavior, though fs-routes is documented as experimental and not subject to semver.

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https://claude.ai/code/session_0183g2LXhE2PfPpg5KTmFLrM

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