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

apps/solidcraft-web/src/files.rs

@@ -376,7 +376,13 @@ pub fn autosave(files: &Rc<Files>, doc: Option<(&solidcraft_engine::doc::Documen
     let files = files.clone();
     match doc {
         Some((d, path)) => {
-            let bytes = solidcraft_engine::io::write_design(d);
+            let bytes = match solidcraft_engine::io::write_design_container(d) {
+                Ok(bytes) => bytes,
+                Err(e) => {
+                    files.say(format!("autosave: {e}"), true);
+                    return;
+                }
+            };
             let meta = serde_json::json!({"name": d.name, "path": path, "saved_at": js_sys::Date::now()}).to_string();
             spawn_local(async move {
                 let r = async {
@@ -452,7 +458,7 @@ fn input_picker(files: &Rc<Files>, intent: Intent) {
     let Some(doc) = web_sys::window().and_then(|w| w.document()) else { return };
     let Ok(input) = doc.create_element("input").map(|e| e.unchecked_into::<web_sys::HtmlInputElement>()) else { return };
     input.set_type("file");
-    input.set_accept(".solidcraft,.solvecraft,.step,.stp,.3mf,.stl");
+    input.set_accept(".solidcraft,.solvecraft,.json,.step,.stp,.3mf,.stl");
     // Attached (hidden) while it is open: some browsers ignore a click on a detached input.
     let _ = input.set_attribute("style", "display:none");
     if let Some(body) = doc.body() {

apps/solidcraft-web/src/main.rs

@@ -251,8 +251,10 @@ mod web {
                 },
                 "download_open" => {
                     let d = &self.app.session.doc;
-                    let bytes = solidcraft_engine::io::write_design(d);
-                    if let Err(e) = files::download(&format!("{}.{}", d.name, solidcraft_engine::io::DESIGN_EXT), &bytes) {
+                    let result = solidcraft_engine::io::write_design_container(d)
+                        .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 {
                         self.app.set_status(e, true);
                     }
                 }