`router`: Switch does not support Fragment-wrapped children or fallback routes

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*This issue was written by an AI assistant (Claude) on behalf of [@silverwind](https://github.com/silverwind).*

The current `Switch` implementation requires a flat array of `Route` elements and calls `.find()` directly on `props.children`. This has two limitations:

### 1. Fragment-wrapped children are not supported

When routes are grouped inside `<>...` fragments (common when conditionally rendering groups of routes), `Switch` sees the Fragment element itself rather than the Route children inside it.

```jsx

<>

```

In this example, the Fragment is treated as a single child and `child.props.path` is `undefined`, so the routes inside it are never matched.

### 2. No fallback/catch-all route

A child without a `path` prop cannot act as a fallback. The current logic calls `matchRoute(child.props.path, path)` which returns falsy for `undefined`, so pathless children are always skipped. A common pattern is to place a fallback element as the last child of `Switch` without a `path`:

```jsx

```

### Suggested behavior

- Recursively flatten Fragment children before matching
- Treat a child without a `path` prop as a catch-all that matches any route

This would align with how React Router's `Switch` historically worked and is a common expectation.

Start at the Switch implementation described in the issue and inspect how props.children is currently traversed and matched. Verify the change against the fragment-wrapped routes and pathless fallback examples in the issue, then run the project's existing tests to confirm both route forms work.

Written by the indexing model from the issue text.