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

docs/mcp.md

@@ -185,6 +185,27 @@ edges of partition cells matching counts.
 4. `mesh.seed.list` lists the seeds and the edges they reach (`stale` when the design has no such
    edge); `mesh.seed.clear {edges?}` removes them. Seeds are saved with the model.
 
+## Size controls
+
+Ask for a local element size where the answer is decided (a fillet, a hole's wall, a contact
+patch) instead of making the whole part fine.
+
+1. `execute` `mesh.size.set {name, size, faces: [{body, face}] | bodies: [...] | box: {min, max} |
+   sphere: {center, radius}}` adds the control (or replaces the one of that name). Faces are
+   named as `inspect_model` lists them.
+2. `mesh.size.growth {rate}` sets how fast the size grows away from a control (the largest
+   change of element size per unit length; 0.3 by default, 0.05 to 2).
+3. Mesh: every mesher honours the controls. The size at a point is the smallest of the mesh
+   command's `size` and, over the controls, a control's size plus the growth rate times the
+   distance to it. The tet mesher refines to that field; the structured hex meshers, the block
+   projection and mesh plans divide their edges by it (a structured mesh carries a count across
+   its block, so the refinement spans the row or layer); the octree refines its cells to it (a
+   size that needs more levels than `max_depth` is refused before meshing). A control on whole
+   bodies is their size in place of the command's.
+4. `mesh.size.list` lists the controls, what each covers in the open design (`stale` when the
+   design no longer has a face it names) and the growth rate; `mesh.size.clear {names?}` removes
+   them. Controls are saved with the model.
+
 ## Meshing from the feature graph
 
 A design opened from `.solidcraft` is rebuilt from its timeline, so how each body was made is