[popover] Allow `Positioner` parts to be omitted for native CSS anchoring

When trying to use native CSS anchor positioning by simply omitting the `Positioner` part to position `Popup` with CSS `anchor`, an error is thrown:

> Base UI: PopoverPositionerContext is missing. PopoverPositioner parts must be placed within .

Native CSS Anchoring doesn't support `Arrow` yet, but it can optionally consume the context instead of throwing for future use. `Popup` doesn't need to consume the positioner context at all (`FloatingFocusManager` can work with `FloatingRootContext` from the root instead).

This way, the Floating UI logic doesn't need to be included in the bundle once CSS anchoring is widespread if they just omit the `Positioner` part in their JSX.

Start by tracing the Popover.Positioner and Popup parts, especially PopoverPositionerContext and FloatingFocusManager. Check how omitting Positioner currently triggers the missing-context error, then verify that CSS anchor positioning works without Floating UI while Arrow can consume the context when present.

索引モデルが issue の本文から書いたものです。