The .modelcraft file, undo/redo on the journal, atomic saves and autosave recovery (M3) (#21)

* modelcraft-io: the .modelcraft file (modelcraft/1), atomic writes, autosave storage

A stored ZIP of a manifest (format tag, geometry sources with SHA-256,
part sizes and hashes), the model definition as stable pretty JSON, the
mesh and geometry tags in a compact binary part, and the journal one
command per line. Reading caps every size, checks every checksum and
replays the model through its checked methods. Model::without_mesh and
empty mesh fields left out of the model's JSON; geometry::read_file.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: undo/redo on the journal, file.open/save/save_as, autosave and recovery

CommandSpec gains a kind: Model commands are journalled and become undo
steps; Session commands (file.*, edit.*) manage the session. Undo history
is bounded by steps and estimated memory. Geometry sources are recorded
with content hashes and reopened on file.open (missing or changed files
are warnings). Desktop app autosaves every 30 s and reports recoverable
models; Ctrl/Cmd+Z and Shift+Z/Y undo and redo.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: the .modelcraft format, undo history, roadmap and parity status

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* simplify: shared path parameter, one load path for open/recover, history accounting in History

Session commands skip the per-command model copy; saved revision is an
Option; journal written from borrowed entries and read without an
intermediate tree; backups copied by the file system; recovery lock and
error helpers; geometry file wrappers use read_file.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -25,7 +25,7 @@ crates in strictly lower layers.
 | L1 | `modelcraft-decks` | Abaqus input, Nastran bulk data and LS-DYNA keyword readers and writers, to and from `model`. | planned |
 | L1 | `modelcraft-render` | Display data without a GPU dependency: the visible faces, element edges and nodes of a model, the orbit camera, a CPU rasterizer for headless snapshots; contour maps later. | started |
 | L2 | `modelcraft-doc` | The document: geometry, mesh, model, results, history. | planned |
-| L3 | `modelcraft-io` | `.modelcraft` files, result files. | planned |
+| L3 | `modelcraft-io` | `.modelcraft` files (docs/formats/modelcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery. Bytes in, bytes out; the file system only in `fs` and `recovery`. Result files later. | started |
 | L4 | `modelcraft-engine` | Commands (`CommandSpec`), `Session::execute` with rollback and the panic guard, scripts. | started |
 | L5 | `modelcraft-ui-egui`, `modelcraft-mcp` | Desktop UI (egui + wgpu viewport, model tree, commands, status bar) and control channel requests (docs/control-protocol.md); MCP server. | ui started |
 | testkit | `modelcraft-testkit` | Test tooling, a dev-dependency only: the meshing test corpus (`tests/corpus/*/recipe.json`, analytic shapes) and its runner (`cargo xtask corpus`). | started |
@@ -125,7 +125,12 @@ element names, cards and keywords):
 Every user-visible action is a `CommandSpec` in `crates/engine/src/cmd/<area>.rs` with a JSON
 form. `Session::execute` clones the model, runs the command under `catch_unwind`, and restores
 the model on an error or a panic; commands that changed the model are journalled, so the
-journal replays as a script.
+journal replays as a script. Each journalled command is also an undo step (the state before it):
+`edit.undo` restores that state and moves the command off the journal onto the redo stack,
+`edit.redo` puts it back, so the journal always rebuilds the current model. History is bounded by
+step count and by an estimate of the memory the kept states hold (oldest steps go first).
+Commands of kind `Session` (`file.*`, `edit.*`) are never journalled. `file.save` and `file.open`
+write and read `.modelcraft` files; the desktop app autosaves unsaved work for `file.recover`.
 
 ## Test corpus