OptionalcurrentExisting override, so the editor opens showing current state rather than empty (the re-edit case).
OptionalisTrue when the element renders as a multi-type pie, which forces its shape (R-8). Derived from the UGM automatically in the convenience form.
OptionallabelDisplay name in the header; falls back to the id.
OptionaltypeOffered as the type scope; omit for node-scoped only.
R-12b (round 21, 2026-08-05): the two facts the editor actually needs about its target. A structural scene carries a StructuralGraphInput rather than a UGM, so the editor could not be opened over one at all; passing a descriptor covers both callers and also lets the canvas case work on a graph the host has not loaded into a UGM.