Tracks whether the host element is currently hovered via mouse pointer.
Designed as a hostDirective composition primitive - attach it to
components that need hover state without implementing their own
mouseenter/mouseleave logic. Host components read the read-only
hovered signal via inject(CngxHoverable, { host: true }); the
pointer events are the only writers.
Used internally by CngxTreetableRow for row highlight-on-hover.