engine: geometry.faces.merge, faces meshed as one (virtual topology, #505)

geometry.faces.merge {name, faces} records a face merge in the model;
it is applied after the partitions whenever the design is opened or cut
again, undoes through the journal and is saved with the model. Meshers,
mesh.plan (features and rings see merge:<name>; a merged face is
prismatic when its B-rep faces are), checks and the viewport see one
face, drawn in the warm colour. geometry.faces.merge.delete removes it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/src/geometry.faces.merge.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 8 L12 4 L21 8 L12 12 Z", "fill": "soft", "stroke": "line"}, {"d": "M3 8 L3 16 L12 20 L12 12 M12 20 L21 16 L21 8", "stroke": "line"}, {"line": [7.5, 6, 16.5, 10], "stroke": "warm", "width": 1.25, "dash": [1.5, 2]}, {"d": "M3 8 L12 4 L21 8 L12 12 Z", "stroke": "accent", "width": 1.75}]}