Docs/tracking: dynamic chunks loading

## Status
Base mechanism already exists on `main`; narrowing this to **docs + remaining tracking**.

Async bundle loading is wired through `setAsyncImport` / `waitForAsyncBundleFx` in the core route and surfaced via `createLazyRouteView` (see #27). Route opening waits for the async chunk before completing.

## What's left
- Document dynamic chunk loading end to end (SSR + client, prefetch, fallback).
- Track any remaining sub-features not covered by `createLazyRouteView`.

## Traceability
Source: atomic-router/atomic-router#78 — https://github.com/atomic-router/atomic-router/issues/78