Clicking a superhref link loses the # part of the address

## What happens

A page address can end with a `#` part. It marks a place on the page.

```
/docs?tab=api#install
```

A superhref link is only the query part:

```
?tab=faq
```

When you click it, the browser keeps the path and swaps the query part. But it also throws away the `#` part. You land on:

```
/docs?tab=faq
```

The `#install` is gone. Nobody asked to remove it. It just disappears.

This happens with every string superhref returns. It also happens with the bare `?` string that clears the query (added in #13).

## What should happen

The `#` part should stay unless someone removes it on purpose. Clicking `?tab=faq` on the page above should land on:

```
/docs?tab=faq#install
```

## Possible fix

`bind()` receives the whole URL, so it knows the `#` part. It could put the `#` part at the end of every string it returns:

```
?tab=faq#install
```

This needs a decision and tests.

Start with bind(), which receives the whole URL and produces the superhref strings; inspect how it handles the query and fragment portions. Add coverage for ?tab=faq and the bare ? when the current URL includes #install, and consider the issue complete when the fragment is preserved in both returned strings.

Written by the indexing model from the issue text.