Open a design of another part: ask whether to start a new model

Opening (File > Open, a drop, the recent list) a design that has none of the
bodies the model's mesh and sets are on now asks, like the unsaved-changes
prompt: Start a new model with this design (default, Enter), Keep the
current model (the status names what no longer resolves), or Cancel. The
opened session waits for the answer; the same design opens without asking.
ui.inspect shows the prompt as {"design": path}; ui.prompt answers it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/lib.rs

@@ -313,6 +313,8 @@ pub struct ModelApp {
     pub queued: std::collections::VecDeque<String>,
     /// The question the window is asking (unsaved changes, recover).
     pub prompt: Option<files::Prompt>,
+    /// A design opened over a model of another part, waiting for [`files::Prompt::Design`]'s answer.
+    pub held: Option<files::Opened>,
     /// The window draws its own title bar and caption buttons (Windows, Linux).
     pub custom_titlebar: bool,
     /// The title bar runs under the macOS traffic lights.
@@ -382,6 +384,7 @@ impl ModelApp {
             loading: None,
             queued: std::collections::VecDeque::new(),
             prompt: None,
+            held: None,
             custom_titlebar: false,
             integrated_titlebar: false,
             palette_text: String::new(),