Add design containers with deduplicated blobs and JSON compatibility (#327)

* Add fast design containers with lossless deduplicated payloads

* Keep display caching opt-in to avoid default save overhead

docs/book/src/guide/files.md

@@ -2,7 +2,8 @@
 
 | Format | Read | Write |
 |---|---|---|
-| `.solidcraft` (JSON design, versioned) | yes | yes |
+| `.solidcraft` (versioned container or JSON design) | yes | yes |
+| `.json` (self-contained plain JSON design) | yes | yes |
 | STEP (AP203, AP214, AP242) | solids, assemblies, units, names, colours | AP242 with names, colours and the component tree |
 | IGES 5.3 | manifold solids, or trimmed surfaces sewn into solids | manifold solids with names and colours |
 | 3MF, STL, OBJ | mesh bodies | yes |
@@ -32,7 +33,26 @@ replaced version is kept as `<file>.bak`. Unsaved changes are autosaved to a rec
 and the next launch offers to recover them. Older design files are upgraded on open, and files
 from a newer version are refused with a message.
 
-Current designs use format 12, including persistent analytic offset boundary roles and
+**Save** writes a container by default; files named `.json` keep JSON on subsequent saves.
+**Save As** offers a container or plain JSON on
+desktop; **Save As JSON** is also available in the File menu on desktop and web. Choose
+the `.json` extension for plain JSON. **Export Design as JSON** writes an independent,
+self-contained copy without changing the current file. Scripts can explicitly pass
+`"format": "json"` to `file.save` or `file.save_as` even with a `.solidcraft` name.
+Opening detects the format from the contents, so existing JSON designs keep working.
+Containers favor fast reading and saving by default. Scripts can request
+`"compression": "compact"` when file size matters more than CPU time.
+
+Containers deduplicate identical STEP, mesh and image payloads. Mesh coordinates retain
+their original double precision; this is not a conversion to STL. Geometry still rebuilds
+from the timeline when opening. When a saved display preview is available, desktop Open
+shows it while checking geometry; editing starts after the complete rebuild. Cancelling
+keeps the current design. Previews cannot be edited or measured. This experimental option
+requires `preview: true` in an engine Save/Save As call; ordinary saves omit display caches.
+The repository's `docs/design-container.md` describes limits and encoding details.
+
+Current designs use format 14, including persistent names from exact face history,
+exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned
 revolve conventions. It also retains recorded body transfers, explicit 3D curve coordinate
 spaces and certified face bindings, alongside semantic configuration styles, associative