ui: jobs with progress, refusals listed, datum preview, form filters and defaults

- A solve or a mesher runs as a job with one progress mechanism: the overlay
  shows the step, stage, count and elapsed time with a bar when bounded, OK
  waits while it runs, and the status sums it up (elements, smallest angle,
  time) (#322, #357).
- Over the control channel such a command is a job too: answered when it
  ends however long it runs, or at once with wait: false, then job.status,
  job.wait, job.cancel; ui.inspect gives job; MCP waits for it (#367).
- A command that cannot run yet lists what it misses in the context panel,
  each with a button to the command that provides it (#323).
- A cancelled or refused solve stays in its workspace (#324, test).
- A mesher form starts from the parameters of the mesh it would replace (#343).
- Seed Edges and edge/vertex/face datum points switch the selection filter
  while open and restore it (#341).
- Datum forms draw the datum live, the Partition form shows its datum, and
  new datums and partitions get the next free name (#347).

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

crates/ui-egui/src/scene.rs

@@ -89,6 +89,16 @@ pub fn datums(s: &mut Scene, session: &fieldcraft_engine::Session, bounds: &Aabb
     fieldcraft_render::glyphs::datums(&session.model, bounds, &shown, selected, &t.glyph_colors(), s)
 }
 
+/// A datum being defined in a form, drawn highlighted (through the part) where OK would put it.
+pub fn datum_preview(s: &mut Scene, datum: fieldcraft_model::Datum, bounds: &Aabb3, t: &Tokens) -> Aabb3 {
+    const NAME: &str = "preview";
+    let mut model = fieldcraft_model::Model::default();
+    if model.add_datum(NAME, datum).is_err() {
+        return Aabb3::EMPTY;
+    }
+    fieldcraft_render::glyphs::datums(&model, bounds, &|_| true, Some(NAME), &t.glyph_colors(), s)
+}
+
 /// The scene for `display` drawn in `style`.
 pub fn build(display: &DisplayMesh, style: Style, show: Show, t: &Tokens) -> Scene {
     let mut s = Scene { background: Some((rgb(t.viewport_top), rgb(t.viewport_bottom))), ..Default::default() };