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 <noreply@anthropic.com>
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 <noreply@anthropic.com>
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 <noreply@anthropic.com>
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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/parity.md

@@ -11,12 +11,12 @@ products are run, no documentation text is copied). Status: **done**, **started*
 | # | Opportunity | Why analysts want it | ModelCraft approach |
 |---|---|---|---|
 | 1 | Hex meshes without manual partitioning | Partitioning a part into sweepable cells by hand is the most cited chore | Sweep and mapped meshing with automatic decomposition that proposes SolidCraft splits (M5); octree all-hex for anything else (M6) |
-| 2 | Undo, redo, replay, crash recovery | No general undo; replay files are hard to reuse; corrupted model files | Every action is a journalled command with rollback |
+| 2 | Undo, redo, replay, crash recovery | No general undo; replay files are hard to reuse; corrupted model files | Every action is a journalled command with rollback, undo and redo; autosave with recovery after a crash |
 | 3 | Sets that survive changes | Sets and loads break when geometry regenerates or is remeshed | Sets bind to SolidCraft's persistent face, edge and body names and resolve after meshing (M3) |
 | 4 | An API that is not a bundled scripting runtime | Version breakage, slow scripts, geometry picked by position | JSON commands over CLI, control channel and MCP |
 | 5 | Located diagnostics | Generic error messages | Errors name the element, face or keyword; `mesh.quality` lists failing elements |
 | 6 | Lossless deck round trip | Unsupported keywords dropped on import | Deck readers keep unknown keywords and write them back (M9) |
-| 7 | Safe, diffable model files | Binary model databases that corrupt or lock to a version | Text `.modelcraft` model, atomic saves, autosave (M3) |
+| 7 | Safe, diffable model files | Binary model databases that corrupt or lock to a version | `.modelcraft`: text, line-diffable definition and journal, checksummed binary mesh, versioned format tag, atomic saves with backup, autosave (M3, done) |
 | 8 | Geometry checked before meshing | Slivers and tiny faces fail late | Surface preparation reports defects with their location (M1) |
 | 9 | No size caps, any solver | Node limits in learning editions; one solver per tool | Free, open source, three deck formats |
 | 10 | Clear step history | Loads and BCs propagate between steps implicitly | An explicit step × load/BC table (M3) |
@@ -66,7 +66,7 @@ products are run, no documentation text is copied). Status: **done**, **started*
 | Results | Readers for public result formats (Nastran OP2, LS-DYNA d3plot, Abaqus `.fil`) | M9 | | planned |
 | Results | Mesh convergence studies | M9 | | planned |
 | App | Desktop viewport, model tree, commands, control channel | continuous | | done |
-| App | Undo/redo on the command journal, autosave, `.modelcraft` file | M3 | | planned |
+| App | Undo/redo on the command journal, autosave, `.modelcraft` file | M3 | | done |
 | App | MCP server, browser build | continuous | | planned |
 
 ## Deck notes for the writers