fix cursor offset inside CSS zoom containers, add syncZoom() API and demo

[x2605]

fix cursor offset when chart is inside a CSS zoom container

When a parent element has CSS zoom applied (e.g., zoom: 1.5), the cursor crosshair and snap point are offset from the actual mouse position. The offset scales proportionally with the zoom factor.

The root cause is that e.clientX - rect.left gives the offset in zoomed (viewport) pixels, but the overlay element's dimensions are in logical (unzoomed) pixels, so the position mapping is wrong.

The fix divides the offset by the effective CSS zoom, which is computed by walking up the DOM tree and accumulating each ancestor's zoom value. This is recalculated in syncRect(), so it stays in sync on mouseenter, scroll, and resize.

For cases where CSS zoom changes dynamically while the cursor is already over the chart, a new u.syncZoom() method is exposed:

• u.syncZoom() — auto-detects from the DOM
• u.syncZoom(1.5) — sets explicitly without DOM traversal

A demo is included at demos/inside-css-zoom.html.

This fix was authored with the help of Claude.