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/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;