engine: mesh.size.* commands, honoured by every mesher (#435)

mesh.size.set adds a size control on faces, bodies, a box or a sphere;
mesh.size.growth sets the growth rate; mesh.size.clear and
mesh.size.list. The tet mesher bakes the controls into its graded field,
the hex meshers, cells and the octree take the graded RegionSize, and
the block projection and mesh plans divide their edges by it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

skills/fieldcraft-meshing/SKILL.md

@@ -111,7 +111,8 @@ Shapes without a timeline (STEP, 3MF) skip `mesh.plan` and go to step 4.
   pure conduction. Set `layers` in `mesh.plan`, or `mesh.seed.edge` across the wall.
 - Matching counts on shared edges: `mesh.seed.edge`.
 - Size from the physics: finer at stress gradients, fillets and contact; in explicit, from the
-  wave length.
+  wave length. Refine locally with `mesh.size.set` (faces, bodies, a box or a sphere) rather
+  than making the whole part fine; `mesh.size.growth` sets how fast it coarsens away.
 
 ## 6. Quality, mark, relax