engine: geometry.face.split, a face split along a datum plane (#508)

geometry.face.split {name, face, datum} records a face split; it is
applied after the merges whenever the design is opened or cut again,
follows its datum (which it holds), undoes, and is saved with the
model. geometry.face.split.delete removes it.

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.
 
-81 ready, 37 coming soon.
+82 ready, 37 coming soon.
 
 ## Geometry
 
@@ -24,6 +24,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Prepare | Partition — Cut bodies into cells by a datum plane; the two faces of each cut are paired by name | ready | `geometry.partition`, (`geometry.partition.delete`), (`geometry.partition.suppress`) |
 | Prepare | Merge Faces — Mesh adjacent faces as one (virtual topology): a fillet in pieces, a face split by an imprint; the B-rep is unchanged | ready | `geometry.faces.merge`, (`geometry.faces.merge.delete`) |
 | Prepare | Absorb Faces — Absorb tiny and sliver faces into a neighbour for meshing, one picked or every one Check Geometry found | ready | `geometry.face.absorb` |
+| Prepare | Split Face — Split a face along a datum plane for meshing only: two faces on its surface, the cut an edge between them | ready | `geometry.face.split`, (`geometry.face.split.delete`) |
 | Prepare | Stitch — Close an imported surface's gaps under a tolerance: open points welded, T-junctions split | ready | `geometry.stitch`, (`geometry.stitch.delete`) |
 | Prepare | Midsurface — Extract midsurfaces of thin parts for shell meshes | coming soon (M8) |  |