engine, ui: mesh.node.relax and relaxation fields, Relax Nodes tool with sphere and preview (#398)

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

docs/features.md

@@ -6,7 +6,7 @@ Every entry is a toolbar button. **Ready** entries run their command; **coming s
 shown greyed with the roadmap milestone that brings them (ROADMAP.md). Scripts, the command line,
 the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 
-70 ready, 39 coming soon.
+71 ready, 39 coming soon.
 
 ## Geometry
 
@@ -50,6 +50,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Edit | Element — Add an element on existing nodes | ready | `mesh.element.add`, `mesh.element.remove`, `mesh.clear` |
 | Edit | Mirror Mesh — Mirror a body's mesh across a plane, joined on the plane (mesh half a symmetric part) | ready | `mesh.mirror` |
 | Edit | Merge Nodes — Merge coincident nodes, keeping sets, loads and boundary conditions on the survivors; check for duplicates and cracks, renumber | ready | `mesh.node.merge`, `mesh.check.duplicates`, `mesh.renumber` |
+| Improve | Relax Nodes — Relax the nodes inside a sphere to fix spikes and slivers; surface nodes slide on the geometry; kept as a field to apply again after a remesh | ready | `mesh.node.relax`, `mesh.node.relax.apply` |
 | Edit | Remesh Region — Remesh a region in place; split and collapse elements | coming soon (M6) |  |
 | Edit | Orphan Mesh — Import a mesh without geometry | coming soon (M6) |  |
 | Verify | Quality — Quality metrics and the certified validity check, failing elements listed | ready | `mesh.quality` |