Attach active src route to next global (#90171)
## Summary
- expose the active app source page on `window.next.__n_src_page`
- derive the source page from the client router tree instead of sending
it via `InitialRSCPayload`
- ensure the value updates on client transitions (including dynamic
routes and route groups)
- extend app-dir e2e coverage for initial load and transition behavior
## Testing
- `pnpm test-dev test/e2e/app-dir/app/index.test.ts -t "__n_src_page"`
- `pnpm --filter=next types`
