Explore replacing cdk focus trap with native HTML inert

The new HTML `inert` attribute is a native solution to the very manual focus trapping we employ today. We should explore whether we can drop the entire focus trap subpackage in favor of people directly using `inert`. It will be some time until browsers fully support this, but it's on track for all of Chromium/Firefox/Safari.

See
https://developer.chrome.com/blog/inert/
https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/inert
https://caniuse.com/mdn-api_htmlelement_inert

Start with the CDK focus trap subpackage and compare its current manual trapping behavior with the native HTML inert API described in the linked browser documentation. Check the stated Chromium, Firefox, and Safari support, then determine whether the subpackage can be replaced without breaking existing component usage; done means a documented feasibility decision and migration scope.

Written by the indexing model from the issue text.