fix: report a clear error when a fs-routes page module is marked "use client"

[k35o]

Closes #173

Summary

A page module marked "use client" used to fail the build with an error thrown from inside the RSC runtime, with no frame pointing at a source file. collectStaticPaths now detects the client reference and reports which file to fix, and the other tree.ts errors name the file as well.

Changes

• tree.ts: detect a client reference via $$typeof === Symbol.for("react.client.reference") next to the existing typeof guard, and include the source file in the error messages when the tree node carries one.
• types.ts: add optional filePath to FsRouteTreeNode so adapters can pass the source file through for diagnostics.
• nextAdapter.ts: record each page and layout file on the emitted tree nodes.
• FileSystemRouting.mdx: document that a page module exporting generateStaticParams must be a Server Component, with the split-and-re-export pattern.

The new error:

Dynamic route "/blog/:slug" ("blog/[slug]/page.tsx") exports generateStaticParams()
from a module marked "use client". generateStaticParams() runs on the server at build
time, so a page module cannot be a Client Component. Move the component body into a
separate "use client" module and re-export it from the page:
export { default } from "./_page";

The two existing tree.ts errors gain the file name when it is known:

- Dynamic route "/blog/:slug" has no generateStaticParams() export. ...
+ Dynamic route "/blog/:slug" ("blog/[slug]/page.tsx") has no generateStaticParams() export. ...

- generateStaticParams() for "/blog/:slug" is missing a value for param "slug".
+ generateStaticParams() for "/blog/:slug" ("blog/[slug]/page.tsx") is missing a value for param "slug".

filePath is optional, so hand-built trees and custom adapters that do not set it keep the previous messages.

Tests

• tree.test.ts: the client-reference error, the file name in the two existing errors, and that a client component page on a static route stays allowed.
• nextAdapter.test.ts: buildRoutes records filePath on page and layout nodes.

Ran vitest run (120 tests), tsc --noEmit, oxlint, prettier --check, and the package build locally; all pass.

[uhyo]

Thank you for the PR!! 😃🙏