## Goal
Remove the experimental.advancedRouting flag and enable the feature by
default. Move fetchFile to a top-level config option.

RFC: withastro/roadmap#1344

## Decisions
- fetchFile becomes a top-level config option (string | null, default 'app')
  instead of being nested under experimental.advancedRouting. The boolean
  gate is gone — the vite plugin always resolves src/app.ts if it exists.
- Users who had experimental.advancedRouting: true can simply delete it.
- Users who had experimental.advancedRouting: { fetchFile: 'fetch.ts' }
  move to the top-level: fetchFile: 'fetch.ts'.

## Changes
- base.ts: Moved default and zod schema from experimental to top-level.
- config.ts: Moved type from experimental to top-level with JSDoc.
- vite-plugin.ts: Reads settings.config.fetchFile directly, removed
  advancedRoutingEnabled gate.
- Example config: Removed experimental block.
- reference/handlers.md: Removed experimental mention.

The fixture has a src/app.ts that is a Vue app entrypoint, not an Astro
fetch handler. With advanced routing always enabled, it was being loaded
as the fetch handler, breaking the SSR test.

Co-authored-by: Armand Philippot <[email protected]>