Trace route module preparation (#97295)
## Summary
- trace the full route-module preparation boundary as
`RouteModule.prepare`
- record manifest loading as one aggregate `RouteModule.loadManifests`
child span
- keep route preparation visible by default while exposing manifest
detail only with verbose tracing
- cover normal and direct-entrypoint server lifecycles without relying
on test ordering
## Why
Route preparation is the stable boundary between loading a route module
and executing it. The parent span shows the total setup cost; the
aggregate manifest child explains manifest work without creating
per-file spans or exposing paths.
## Verification
- `pnpm build-all`
- `pnpm --filter=next build`
- `pnpm --filter=next types`
- `pnpm exec jest packages/next/src/server/lib/trace/tracer.test.ts
--runInBand`
- focused OpenTelemetry E2E in development and production with Turbopack
and Webpack
- Request Insights route-preparation E2E with Turbopack and Webpack
- Prettier, ESLint, and `git diff --check`
## Stack
Depends on **Trace route module loading**.
