exportparts — forwarding parts up through nesting

Expected: "Label:" is blue and bold, and "42" sits on a yellow band — both live two shadow boundaries deep, and reach the document only because the middle component re-exports them with exportparts.

Renaming while forwarding

Expected: "Renamed label:" is underlined (it is exported as inner-lbl) and not blue and bold — the outer document's ::part(label) rule no longer reaches it under the old name. The value is still yellow-banded, exported under its own name.

Parts not listed are not forwarded

Expected: "Label:" is blue and bold; the value has no yellow band, because exportparts lists only label.