[select] popup is not removed from the DOM after closing.

# Bug report

## Current behavior

After the Select's popup has been closed, it remains in the DOM.

## Expected behavior

The popup should be removed from the DOM

## Reproducible example

Inspect the doc's first example

https://base-ui.com/react/components/select

## Base UI version

v1.6.0

## Which browser are you using?

Chrome

## Which OS are you using?

Mac OS

## Which assistive tech are you using (if applicable)?

## Additional context

Start with the Select component used by the first example on the documentation page at https://base-ui.com/react/components/select and reproduce the behavior in Chrome by inspecting the DOM before and after closing the popup. Trace the popup lifecycle and verify that closing the Select removes the popup from the DOM.

Written by the indexing model from the issue text.