UI: seeds as ticks along their edges, Mesh › Seeds, and seeding the selection

A seed draws a tick at each node it puts on its edge, so a bias shows as ticks
crowding toward an end. The browser lists the seeds under Mesh; a click
selects the seeded edge. The Seed Edges form takes the selected edges with
Use selection. The toolbar entry is ready (its icon renamed with it).

Part of #230.

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

assets/icons/src/mesh.seed.edge.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 19 C7 5 17 5 21 19", "stroke": "line"}, {"circle": [3, 19, 1.3], "fill": "accent", "stroke": null}, {"circle": [6.024000000000002, 12.280000000000001, 1.3], "fill": "accent", "stroke": null}, {"circle": [9.912000000000003, 8.919999999999998, 1.3], "fill": "accent", "stroke": null}, {"circle": [14.088000000000001, 8.92, 1.3], "fill": "accent", "stroke": null}, {"circle": [17.976000000000003, 12.280000000000001, 1.3], "fill": "accent", "stroke": null}, {"circle": [21.0, 19.0, 1.3], "fill": "accent", "stroke": null}]}