[context menu] Right-click open is marked as instant

# Bug report

## Current behavior

Opening a Context Menu with a mouse right-click adds `data-instant="click"` to the Popup. A right-click `contextmenu` event has `detail === 0`, so the shared Menu logic classifies it as a keyboard click.

As a result, consuming the documented attribute with `data-instant:transition-none` skips the enter transition, while a subsequent mouse close still plays the exit transition.

## Expected behavior

A mouse right-click should not be classified as an instant keyboard click. The `data-instant` value should accurately describe the interaction that opened the Context Menu.

## Reproducible example

Add `data-instant:transition-none` to the Popup in the [official Context Menu demo](https://base-ui.com/react/components/context-menu), then open it with a mouse right-click. The enter transition is skipped.

## Base UI version

1.7.0

## Which browser are you using?

Chrome

## Which OS are you using?

macOS

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

Not applicable.

## Additional context

`ContextMenu.Popup` re-exports `MenuPopup`, and the Context Menu demos do not currently consume `data-instant`, although it is listed in the Popup API.