Route Handler Code No Longer Preloaded On Next Start Since 16.3

Since Next.js 16.3, AppRouteRouteModule wraps userland code in LazyModule (_lazyUserland). The startup preload mechanism (unstable_preloadEntries in next-server.ts) calls loadComponents() but does not trigger routeModule.ensureUserland(), leaving module evaluation deferred until first request.

1. Create a Next.js app with an app/api/heavy/route.ts that spends 500ms at module evaluation. 2. Run next build and next start. 3. Wait for startup (10s) then time two GET /api/heavy requests. Observe first request ~500ms, second ~2ms on affected versions.

This fix increases startup time and memory usage as all route handlers are fully evaluated at boot, which may be undesirable for apps with many huge route modules. In concurrent or multi-threaded environments, ensure that preloading is idempotent and does not conflict with lazy loading assumptions. If rollback is needed, revert this change and rely on runtime lazy loading; consider making preloading configurable via experimental flag.