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

crates/ui-egui/src/context.rs

@@ -100,6 +100,7 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
             let tip = match target {
                 selection::Target::Members => "The selection becomes the set's members (geometry by name, the mesh by label)",
                 selection::Target::Set | selection::Target::TargetSet => "Makes a set of the selection (model.set.create) and names it here",
+                selection::Target::Edges => "The selected design edges (Select: Edges) become the edges seeded",
             };
             use_selection = ui.add_enabled(selected.is_some(), egui::Button::new("Use selection")).on_hover_text(tip).clicked();
             match &selected {