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/mesh.seed.edge.svg

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="24" height="24" viewBox="0 0 24 24">
-  <title>mesh.size.seed</title>
+  <title>mesh.seed.edge</title>
   <g id="Layer_1" data-name="Layer 1">
     <path d="M3 19 C7 5 17 5 21 19" fill="none" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
     <path d="M1.7 19 C1.7 18.282 2.282 17.7 3 17.7 C3.718 17.7 4.3 18.282 4.3 19 C4.3 19.718 3.718 20.3 3 20.3 C2.282 20.3 1.7 19.718 1.7 19 Z" fill="#4f9ce8"/>