Explain traceability views and preserve cross-layer placement context

crates/oa-desktop/src/views.rs

@@ -599,6 +599,28 @@ fn diagram(app: &App) -> View<'_, Message> {
     }
     if let Some(diagram) = app.active_diagram {
         let view = oa_core::projection::project_view(app.project(), diagram);
+        if projection == Some(ProjectionKind::Traceability) {
+            let correspondence_lines = view.edges.iter().filter(|edge| edge.trace).count();
+            tools = tools.push(
+                column![
+                    label("Shows OA and SA elements together, with correspondence lines alongside ordinary relationships."),
+                    label("Place both ends of a trace to show its line. Port traces use their owner cards; traces between relationships are reviewed on the Traceability page."),
+                    row![
+                        label(format!("{correspondence_lines} correspondence lines in this view")),
+                        action(
+                            "Add existing elements…",
+                            Message::Architecture(crate::app::architecture_authoring::Event::Existing(None))
+                        ),
+                        action("Review layer traces", Message::ReviewLayerTraces),
+                        action("View visibility…", Message::Select(diagram)),
+                    ]
+                    .spacing(7)
+                    .align_y(iced::Center)
+                    .wrap(),
+                ]
+                .spacing(4),
+            );
+        }
         let edge = view.edges.iter().find(|edge| edge.id == app.selected);
         let promoted = edge.is_some_and(|edge| {
             oa_core::projection::edge_endpoint_placements(