Save prompt on quit: "Save changes to <name>?" for every unsaved design

The title bar's ×, File ▸ Quit, the OS closing the window (Alt+F4, Cmd+Q, the window manager:
close_requested is cancelled until the designs are answered) and app.quit / ui.window close all
go through documents::request_quit, which asks Save / Don't Save / Cancel once per design with
unsaved changes. Save saves (Save As for a never-saved design) and quits only if it saved; Don't
Save discards (no recovery copy is kept); Cancel or Esc stays open. Closing a tab asks the same
question (now titled "Save changes to <name>?"); New and Open never replace an unsaved design
(they open a tab). The web app asks through beforeunload while a design is unsaved. Control:
ui.documents {action: quit, answer: save|dont_save|cancel}, app.quit {force}. Tests: quit with
Save then Don't Save across two tabs, Cancel, a Save that saves nothing, Save As; scenario
s43_quit_asks_to_save.

Fixes #33

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/scenario.rs

@@ -738,6 +738,11 @@ pub fn check(h: &mut Harness, e: &Value) -> Result<(), String> {
                     return Err(format!("start page open: got {}, want {v}", ui["home"]));
                 }
             }
+            "quit_requested" | "quitting" => {
+                if ui[k] != *v {
+                    return Err(format!("{k}: got {}, want {v}", ui[k]));
+                }
+            }
             "documents" => {
                 if ui["documents"] != *v {
                     return Err(format!("open designs: got {}, want {v}", ui["documents"]));