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

crates/mcp/src/tools.rs

@@ -224,8 +224,11 @@ pub fn tool_definitions() -> Vec<Value> {
         tool(
             "save",
             "Save the design",
-            "Save the design as a .solidcraft file (to `path`, or to the file it came from).",
-            obj(json!({"path": string("Path to save to (default: the current file)")}), &[]),
+            "Save the design to `path` or its current file. The default is a design container; a .json path selects plain JSON. Set format to container or json to override the path.",
+            obj(
+                json!({"path": string("Path to save to (default: the current file)"), "format": {"type": "string", "enum": ["container", "json"], "description": "Design encoding (default: JSON for .json, container otherwise)"}}),
+                &[],
+            ),
             false,
             false,
         ),
@@ -378,7 +381,7 @@ fn dispatch(b: &mut dyn Backend, name: &str, a: &Map<String, Value>) -> Result<V
         "redo" => exec(b, "edit.redo", json!({})),
         "new_design" => exec(b, "file.new", pick(a, &["name"])),
         "open" => exec(b, "doc.open", pick(a, &["path"])),
-        "save" => exec(b, "file.save", pick(a, &["path"])),
+        "save" => exec(b, "file.save", pick(a, &["path", "format"])),
         other => Err(format!("unknown tool `{other}`")),
     }
 }