[visual-qa] EditVertex drag preview of GeometryCollection(CIRCULARSTRING) is chords not arcs

[grootstebozewolf]

Part of #68.

Observed

GEOMETRYCOLLECTION of CIRCULARSTRING: EditVertex drag shows a red chord preview (lines to adjacent controls), not the arc. A top-level CIRCULARSTRING / COMPOUNDCURVE already rubber-bands via CurveShapeWriter.

Cause: EditVertexTool.circularStringPreviewShape gates on getGeometryType() equal to CircularString or CompoundCurve. GeometryCollection fails the gate and falls through to adjVertices chords.

Expected

Drag preview of a GeometryCollection (or MultiCurve) that contains CircularString / CompoundCurve / ClothoidSegment uses CurveShapeWriter arcs, same as a bare CircularString.

Not

• Not type-flatten on move ([visual-qa] EditVertex CIRCULARSTRING last-point nudge flattens to LINESTRING #74).
• Not insert locater ([visual-qa] EditVertex right-click add vertex on CIRCULARSTRING makes even control count #82 follow-on).

[grootstebozewolf]

PR #102: EditVertex curve preview walks GeometryCollection / MultiCurve instead of a top-level type-name gate. Do not close without UX SIGN.