Function elementToOverlayRect

Convert a Chart.js element (bar, point, etc.) into an overlay rectangle.

Element types expose different geometry properties:

• Bar: x, y, base, width, height, horizontal (read directly).
• Candlestick / OHLC: x (center), width, high/low (top/bottom pixel y), open/close (body endpoints in pixels).
• Matrix (heatmap cell): x, y (center), width, height, no base.
• Point (scatter / line vertex): x, y, plus an effective radius.

Returns null if the element does not expose enough geometry to render.