Implement anchored ContextMenu positioning and update documentation

[akoweicollinxx]

This PR updates ContextMenu to use anchored/context positioning instead of fixed cursor coordinates.

The menu is now anchored to the trigger element, which means it:

scroll-follows its target instead of detaching when the page moves
can auto-flip/shift near viewport edges via the anchored layer behavior
preserves the existing focus restoration and Escape-dismiss behavior
What changed
switched ContextMenu from useLayer({ mode: 'fixed' }) to useLayer({ mode: 'context' })
removed fixed x/y positioning from the menu render path
kept right-click open behavior, focus restore, and keyboard navigation intact
Updated ContextMenu docs to describe anchored behavior
Updated tests to match the new anchoring model
Notes
This is a follow-up to the earlier invocation work that intentionally kept fixed positioning. It completes the anchoring follow-up tracked in #3465.

Testing
Updated unit tests for ContextMenu behavior
verified open/close, Escape handling, and focus restoration still pass

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