Allow applications to choose how the PDFViewer determines the current page during continuous scrolling.

Currently, the page indicator follows the viewport's scroll position without considering its height. With zoom="fitToWidth" in a wide, short viewer, most of the visible content can belong to the next page while the pager still indicates the previous page. The pageChange event follows the same detection rule.

Introduce an optional input, for example:

currentPageThreshold: 'top' | 'center'

• 'top': Preserve the existing detection behavior. Proposed default for backward compatibility.
• 'center': Determine the current page using the vertical center of the document viewport.

Proposed usage; this input is not currently available:

Center-based detection should update the current page earlier when the next page occupies most of a short viewport. It should not be described as a general largest-visible-area algorithm, particularly when multiple pages fit within the viewport.