total height (recursive for composites like group/nested)
Optionalminoptional — the minimum height needed to start rendering, for a block that splits itself across pages (e.g. a breakable box: just its first child + padding). The engine's render loop (and StackBlock's per-child loop) uses this instead of measureHeight for the break-before decision, so such a block can start in the space left on the current page and continue on the next, instead of being pushed whole to a fresh page. Absent = the block needs its full measureHeight up front (every non-splitting block).
draw onto the doc; must not compute page-breaks itself — use ctx.ensureSpace
The shared contract for every block — it must answer two things: how tall, and how to draw. text/image/table/group/report all implement this same interface.