delegates to deriveMeasureContext — MeasureContext construction logic lives in one place (single source)
Draws the page header/footer band on every page — must be called once, after every block's render() has finished (the band is drawn now rather than on page-break, because AutoTable creates pages of its own, bypassing our onPageBreak — iterating every page in the real doc at the end covers every page no matter who created it, and pageCount is known by then). No-op if there's no band; safe to call even without one — the facade/user can always call it.
renders in order: measure → ensureSpace (auto page-break) → render keep-together at the composite level is the block's own responsibility (recursive measureHeight)
Wraps a single jsPDF doc — manages the cursor/page-breaks centrally, injects context into every block. Blocks must never call doc.addPage() themselves; request space only through ctx.ensureSpace.