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.

apps/solidcraft-web/src/main.rs

@@ -251,7 +251,10 @@ mod web {
                 },
                 "download_open" => {
                     let d = &self.app.session.doc;
-                    let result = solidcraft_engine::io::write_design_container(d)
+                    let result = self
+                        .app
+                        .session
+                        .design_container(solidcraft_engine::io::DesignCompression::Fast, false, false)
                         .map_err(|e| e.to_string())
                         .and_then(|bytes| files::download(&format!("{}.{}", d.name, solidcraft_engine::io::DESIGN_EXT), &bytes));
                     if let Err(e) = result {