ui: name a sample's document after it until saved (#334); "Solve cancelled"

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

crates/engine/src/cmd/file.rs

@@ -106,6 +106,7 @@ fn open_sample(s: &mut Session, p: &Value) -> Result<Value> {
     // A sample just opened has nothing to save: it opens again the same way.
     opened.saved_revision = Some(opened.revision);
     opened.saved_view = opened.view.clone();
+    opened.sample = Some(sample.label);
     s.replace_with(opened);
     let mut reply = summary(s, &[]);
     reply["sample"] = json!(sample.name);