Traps keyboard focus within the host element using the CDK FocusTrap.
When enabled, Tab and Shift+Tab cycle only within the host element's
focusable children. When autoFocus is true (default), focus moves to
the first tabbable element automatically on activation.
Wraps CDK's FocusTrapFactory with a declarative, Signal-driven API.
The CDK's imperative create() / destroy() / enabled setter lifecycle
is handled internally via effect() and DestroyRef.