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/engine/src/cmd/datum.rs

@@ -342,6 +342,18 @@ fn datum_of(s: &Session, kind: &str, p: &Value) -> Result<Datum> {
     }
 }
 
+/// The datum a datum form's `params` define so far, for drawing it before OK; `None` when
+/// `command` makes no datum or the parameters do not define one yet. The model is not changed.
+pub fn preview(s: &Session, command: &str, params: &Value) -> Option<Datum> {
+    let kind = match command {
+        "geometry.datum" | "geometry.datum.edit" => "plane",
+        "geometry.datum.axis" | "geometry.datum.axis.edit" => "axis",
+        "geometry.datum.point" | "geometry.datum.point.edit" => "point",
+        _ => return None,
+    };
+    datum_of(s, kind, params).ok()
+}
+
 /// The parameters of the command that edits datum `name`, describing it as it is (a plane by
 /// its point and normal, an axis by its point and direction, a point by its coordinates); and
 /// that command.

crates/engine/src/cmd/mod.rs

@@ -15,6 +15,7 @@ mod hex;
 pub(crate) mod mesh;
 
 pub use datum::edit_params as datum_edit_params;
+pub use datum::preview as datum_preview;
 pub use mesh::analytic_surfaces;
 mod mirror;
 mod model;