fix: fail the build on fs-route configurations that would render broken pages

[uhyo]

Summary

An audit of the experimental file-system router found several inputs that build successfully but produce silently broken output. This PR validates them at build time with clear errors instead. Each case was verified against the real URLPattern behavior that @funstack/router matches with.

generateStaticParams() value validation (tree.ts)

• Values must be non-empty strings without ./.. segments, ?, or #; only catch-all values may contain slashes (and no leading, trailing, or repeated ones). Anything else generated a page its own route could never match — e.g. { slug: "a/b" } for /blog/:slug builds blog/a/b.html, but :slug matches a single segment, so the page renders broken. A .. value could even write HTML outside the output directory.
• Non-string values (e.g. numeric ids) are rejected: the URL would work on a hard load, but the baked params key ([5]) would never match the live key (["5"]), breaking soft navigation.
• An empty catch-all value gets a dedicated error suggesting a page for the parent route: its zero-segment page cannot be soft-navigated to, because the live match yields no param value at all.
• Errors name the route path and source file.

Segment validation in nextRoutes() (nextAdapter.ts)

• Param names must match [A-Za-z0-9_$]+ — what URLPattern can express. [foo-bar] was silently parsed as param foo + literal -bar, producing a route that never matches its own generated pages.
• A param name may be used only once per route path: within one emitted path, URLPattern construction throws at match time; across a layout boundary, the inner value shadows the outer.
• Static directory names must not contain URLPattern metacharacters (:, *, ?, +, parens, braces, backslash), which either throw at match time (?, +) or silently match the wrong URLs.

Other fixes

• A page or layout module without a default export now fails the build instead of silently rendering a blank page (or a pass-through layout).
• fsRoutes.dir must be a subdirectory of the Vite root; . or ../… values previously produced malformed glob patterns (//**/…) that matched nothing.

Cleanups

• FsRouteModule["default"] collapsed to a single ComponentType<FsRouteComponentProps> (the other union members were redundant under component contravariance), dropping two casts in runtime.tsx.
• Removed a stale comment above filesByDir in the adapter.
• Documented the new validation rules in FileSystemRouting.mdx and the generateStaticParams JSDoc.

Test plan

• 14 new unit tests covering every rejected input (142 total passing)
• pnpm typecheck, pnpm lint, pnpm format:check clean
• Full monorepo pnpm build succeeds
• All 17 fs-routing e2e tests pass against a production build

🤖 Generated with Claude Code

https://claude.ai/code/session_01CnAGg4Hh4Fmuu9ULDkdDd2

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