Engine: geometry.datum.point; planes and axes through datum points; datum edits and cascading delete

A datum point by coordinates, at a mesh vertex, a fraction along a
design edge, at a face's centre, or where a datum axis crosses a datum
plane. Datum planes take three datum points or a normal to a datum axis;
axes take two datum points. geometry.datum.edit, .axis.edit and
.point.edit redefine a datum of their kind (partitions cut again);
geometry.datum.delete with cascade also deletes the partitions that cut
by it. The ribbon shows Datum Plane, Datum Axis, Datum Point and
Partition, each with its own icon; edit and delete leave the ribbon
(still commands, claimed by their entries).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/src/geometry.datum.axis.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M7 17 L19 5", "stroke": "accent", "width": 2}, {"d": "M20 4 L15 5.2 M20 4 L18.8 9", "stroke": "accent", "width": 2}, {"d": "M5.5 18.5 L2.5 21.5", "stroke": "line", "dash": [1.5, 2]}, {"circle": [12, 12, 3.2], "fill": "soft", "stroke": "line", "width": 1.25}, {"circle": [12, 12, 1.4], "fill": "warm", "stroke": null}]}