[otel] Add more verbose-mode OTEL spans for App Router (#94894)
This adds two new spans:
- `AppRender.renderToNodeFizzStream`
- `AppRender.waitShellReady`
Using the example from https://github.com/switz/nextjs-tracing-repro
with NEXT_OTEL_VERBOSE:
<img width="1245" height="408" alt="Bildschirmfoto 2026-06-17 um 18 11
28"
src="https://github.com/user-attachments/assets/5bd92d99-fc25-4490-aa25-9f8d0caeaf75"
/>
