feat: add includeTransforms option to measure post-transform rect

[Gabe-Bendix]

This PR adds an opt-in includeTransforms option that measures an element's visual (post-transform) bounds using getBoundingClientRect() instead of the layout box. When enabled, it uses a requestAnimationFrame loop to detect transform-only changes (scale, rotate, etc.) that ResizeObserver misses, while pausing automatically when the tab is hidden. The default behavior remains unchanged for backward compatibility, and the feature only runs when explicitly enabled, incurring zero cost for existing users. This addresses issue #100 by enabling accurate measurements for scaled/transformed elements, useful for tooltips, hit detection, and animation synchronization. The implementation includes tests, documentation, and an interactive demo showing a 40×40 box with scale(0.75) reporting ~30×30 in transform-aware mode vs 40×40 in default mode.
Closes #100