feat: show relationship <name> renders a styled two-table diagram (ADR-0044)

The first wired slice of relationship visualization (V1). `show
relationship <name>` now renders the relationship as two full
structure boxes joined by a width-jogging connector (child-left /
parent-right, n…1 cardinality, on delete/update actions), styled
App-side, with a vertical-stack fallback for narrow terminals.

- db.rs: RelationshipDiagramData + show_relationship worker path
  (structured data: the relationship + both endpoint TableDescriptions)
- runtime.rs: named relationships route to the structured outcome
  (boxed); other show <kind> forms stay prose
- app.rs/event.rs/ui.rs: DslShowRelationshipSucceeded rendered App-side;
  new diagram OutputStyleClass variants; App::last_output_width from ui.rs
- output_render.rs: styled Seg layout engine (boxes, connector routing,
  side-by-side + vertical), composing the ADR-0016 box primitives

Tests: 4 unit + 4 integration; full suite 2201 pass / 0 fail / 1 ignored;
clippy nursery clean. requirements.md V1 stays [/] (show table diagrams,
compound routing, DDL-echo wiring remain).

tests/it/show_list.rs

@@ -357,3 +357,100 @@ fn app_renders_show_list_lines_as_system_output() {
         "item line rendered",
     );
 }
+
+// =================================================================
+// ADR-0044 — `show relationship <name>` renders a diagram
+// =================================================================
+
+#[test]
+fn show_relationship_worker_returns_structured_diagram_data() {
+    let (_p, db, _dir) = open_project_db();
+    let rt = rt();
+    rt.block_on(seed_schema(&db));
+    let data = rt
+        .block_on(db.show_relationship("orders_customer".to_string()))
+        .expect("show_relationship ok")
+        .expect("relationship found");
+    assert_eq!(data.rel.name, "orders_customer");
+    // child = FK holder, parent = referenced (ADR-0044 left/right).
+    assert_eq!(data.child.name, "Orders");
+    assert_eq!(data.parent.name, "Customers");
+    assert_eq!(data.rel.child_columns, vec!["customer_id".to_string()]);
+    assert_eq!(data.rel.parent_columns, vec!["id".to_string()]);
+}
+
+#[test]
+fn show_relationship_worker_returns_none_for_unknown_name() {
+    let (_p, db, _dir) = open_project_db();
+    let rt = rt();
+    rt.block_on(seed_schema(&db));
+    assert!(
+        rt.block_on(db.show_relationship("nope".to_string()))
+            .expect("ok")
+            .is_none(),
+        "unknown relationship → None",
+    );
+}
+
+#[test]
+fn app_renders_show_relationship_as_a_styled_diagram() {
+    let (_p, db, _dir) = open_project_db();
+    let rt = rt();
+    rt.block_on(seed_schema(&db));
+    let data = rt
+        .block_on(db.show_relationship("orders_customer".to_string()))
+        .expect("ok")
+        .expect("found");
+
+    let mut app = App::new();
+    app.output.push_back(rdbms_playground::app::OutputLine::echo(
+        "show relationship orders_customer",
+        Mode::Simple,
+    ));
+    app.update(AppEvent::DslShowRelationshipSucceeded {
+        command: Command::ShowList {
+            kind: ShowListKind::Relationships,
+            name: Some("orders_customer".to_string()),
+        },
+        data: Some(data),
+    });
+    let text: String = app
+        .output
+        .iter()
+        .map(|l| l.text.as_str())
+        .collect::<Vec<_>>()
+        .join("\n");
+    // Both tables, box-drawing, the connector arrow, the actions line.
+    assert!(text.contains("Orders"), "child box: {text}");
+    assert!(text.contains("Customers"), "parent box: {text}");
+    assert!(text.contains('┌') && text.contains('│'), "box drawing: {text}");
+    assert!(text.contains('▶'), "connector arrow: {text}");
+    assert!(text.contains("on delete cascade"), "actions: {text}");
+    // The diagram lines are styled (per-span runs), not plain system.
+    assert!(
+        app.output.iter().any(|l| l.styled_runs.is_some()),
+        "diagram lines carry styled runs",
+    );
+}
+
+#[test]
+fn app_show_relationship_not_found_shows_friendly_line() {
+    let mut app = App::new();
+    app.output.push_back(rdbms_playground::app::OutputLine::echo(
+        "show relationship nope",
+        Mode::Simple,
+    ));
+    app.update(AppEvent::DslShowRelationshipSucceeded {
+        command: Command::ShowList {
+            kind: ShowListKind::Relationships,
+            name: Some("nope".to_string()),
+        },
+        data: None,
+    });
+    assert!(
+        app.output
+            .iter()
+            .any(|l| l.text == "No relationship named `nope`."),
+        "friendly not-found line",
+    );
+}