Opening a design shows the model built so far (#1158)

A large design showed nothing but a progress bar until it had rebuilt completely. While
it rebuilds, the evaluation now offers the model built so far (placed as the design
places it) every 1.5 s to a window that asks, and the open card draws it as a small
picture on a thread of its own, the way it shows a saved preview. The bodies' meshes
made for it are the ones the finished model shows, so little is done twice. A design
that opens within the first interval shows none.

Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/doc/src/eval.rs

@@ -1219,6 +1219,8 @@ impl Model {
                         }
                     }
                     state = Arc::new(next);
+                    // A window opening the design can show what is built so far.
+                    watched.offer(i + 1, doc, &state);
                     None
                 }
                 Err(e) => Some(e.to_string().trim_start_matches("the operation failed: ").to_string()),