fix: cache fs-routes enumeration across dev requests

[uhyo]

Problem

Every dev-server request iterates getEntries(), and the fs-routes implementation did all of its work per invocation. So on every request (HTML and HMR RSC re-fetches alike):

• the route tree was rebuilt and every dynamic route's generateStaticParams() re-ran — user code that often fetches data, making large sites pay the full-site enumeration cost per request;
• one chunk per Server Component node × params combination was re-registered in the dev defer registry under a fresh random ID.

The registration flood interacts badly with the registry's eviction policy (maxPending: 1000, oldest first): a site with C chunks evicts a tab's chunks after ~1000/C further requests. Payloads already served bake the old chunk IDs into their slots, so when a long-idle tab soft-navigates, the chunk fetch 404s and FsRouteChunkBoundary falls back to a hard navigation.

Fix

createFsRoutesEntriesWithHost now enumerates the site once per entries-module instance and caches the result (route tree, pages, node metas, registered chunks); getEntries() iterations reuse it:

• Dev requests no longer re-run generateStaticParams() or re-register chunks; chunk IDs stay stable across requests, so eviction pressure from fs-routes disappears entirely.
• Chunks the dev registry still evicts (the 5-minute TTL on settled entries, or maxPending pressure from defer() calls) are re-registered under their original IDs on the next request, via two new host hooks (hasChunk/restoreChunk), so pages held by open tabs stay soft-navigable.
• HMR keeps working: editing a routed file invalidates the entries module (the virtual module embeds the import.meta.glob), so a fresh module instance re-enumerates.
• Builds are unaffected — they iterate getEntries() once.
• A failed enumeration is not cached, so a transient generateStaticParams() error (e.g. a CMS hiccup) doesn't stick for the rest of the dev session.

Behavior note: generateStaticParams() results derived from external data are now cached in dev until a routed file is edited or the server restarts. Documented in the file-system routing docs.

Testing

• Unit tests added: enumeration runs once across iterations with stable chunk IDs, evicted chunks are restored under their original IDs, and a failed enumeration is retried.
• pnpm typecheck, pnpm lint, pnpm format:check, pnpm test:run all pass.
• Full e2e suites pass: 39/39 (build/preview) and 32/32 (dev server), including all fs-routing dev tests.

This addresses the dev-request cost of the existing enumeration model; a follow-up issue covers the larger improvement of not enumerating the whole site to serve one request (also for the non-fs entries.tsx mode).

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https://claude.ai/code/session_016D8ULaGdQjQ493LBy3ow1S

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