Follow-up: scan node + capture-protocol manifest for scanning-app hand-offs

Follow-up to the seam discussion on #720 (settled there). /import?src= covers the interpreted scene — the editable build graph. The evidence half rides the other seam: the scanning app hosts a capture-protocol manifest (RoomPlan mesh, device-motion camera track, scan video, point cloud — artifacts A3 Atlas already produces), and the exported build JSON carries a scan node whose captureSession locator points at it. Static HTTP source, describe() only — no live streams needed for this use case.

That way one file hands off both: the graph you can edit, and the capture layers that prove it, each on the seam it was designed for. See the #720 discussion for the full reasoning.