Model: datum planes and axes, and partitions by datum plane

Datums are named planes (point, unit normal) and axes (point, unit direction);
a partition names the datum plane that cuts and, optionally, the bodies. A
datum a partition uses cannot be deleted or turned into an axis. Both are
stored in model.json, partitions in the order they cut.

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

docs/formats/fieldcraft.md

@@ -13,7 +13,7 @@ unzip tool reads them: `unzip -p bracket.fieldcraft model.json`):
 | Part | Content |
 |---|---|
 | `fieldcraft.json` | Manifest: `format` (`"fieldcraft/1"`), `generator` (`"FieldCraft 0.5.7"`), `geometry`, and `parts`: the byte size and SHA-256 of each other part. |
-| `model.json` | The model definition: unit system, sets, materials, sections, reference points, couplings, boundary conditions, loads and steps. Pretty-printed JSON with a fixed key order (maps sorted by name), so a change shows as a line diff. Its form is the model's own JSON form, which commands take as parameters. |
+| `model.json` | The model definition: unit system, sets, materials, sections, reference points, couplings, boundary conditions, loads, steps, datum planes and axes, and the partitions that cut the geometry (in order; applied again when the design is opened). Pretty-printed JSON with a fixed key order (maps sorted by name), so a change shows as a line diff. Its form is the model's own JSON form, which commands take as parameters. |
 | `mesh.bin` | Nodes, elements and geometry tags, binary (below). |
 | `journal.json` | The commands that built the model, `[{"command": id, "params": {...}}, ...]`, one per line. It is a script `fieldcraft-cli run` replays. |