MCP: take_screenshot is whole-window only; element-scoped capture needs the rendered rect

Split out from #13244, which reported four verification gaps together.

Problem

take_screenshot captures the whole window. Verifying one element means transferring the entire window and cropping client-side, which in a step-by-step gesture sequence is the dominant cost of the exercise.

Suggested

An optional elementHandle (or an explicit rect) on take_screenshot, cropping server-side before encoding.

Blocked on the rendered rect

Cropping needs the element's extent in window coordinates, which #13250 reports is not currently recoverable: absolutePosition is mapped through the item transform while size is the element's local layout size. Cropping with that pair is correct only when no ancestor scales or rotates the element, and silently wrong otherwise — the same class of bug as #13242.

So this should follow #13250 rather than ship a crop that quietly misses under a transform.

Related

• #13250 — the mapped/unmapped mismatch that has to be resolved first.
• #13244 — the original report of this and three other gaps.