Show a bounded native-cache preview before authoritative reconstruction (#463)

Refs #303. file.save / file.save_as can store a warm native geometry cache (cache: true, off by default) keyed to the document hash, app version and adapter revision. Opening shows a bounded preview from it until the rebuilt session replaces it. Any cache failure counts as a miss.

docs/book/src/guide/files.md

@@ -47,8 +47,11 @@ Containers deduplicate identical STEP, mesh and image payloads. Mesh coordinates
 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.
+keeps the current design. Previews cannot be edited or measured. Scripts can opt in to a disposable geometry
+cache with `cache: true` on warm saves; saving never rebuilds cold geometry just to
+create one. Ordinary saves and browser downloads default to `cache: false`. An additional display-mesh cache remains opt-in
+with `preview: true`. Unsupported or mismatched caches are ignored. Browser opening
+still rebuilds synchronously.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
 Current designs use format 16, including the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis