Payload paginates by page, so floor(offset/limit)+1 alone returned the
wrong window whenever offset % limit != 0 (B9). Fetch the straddled
pages and slice the intra-page remainder. The test stub now honours
page like real Payload (it silently ignored it before), and the
contract pins aligned + non-aligned windows on mock and stub alike.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>