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).

src/ui.rs

@@ -667,6 +667,9 @@ fn render_output_panel(app: &mut App, theme: &Theme, frame: &mut Frame<'_>, area
     // mutable `note_output_viewport` call below).
     let total_wrapped = approximate_wrapped_rows_from_output(&app.output, inner.width);
     app.note_output_viewport(visible, total_wrapped);
+    // ADR-0044 §3: record the panel width so a later `show relationship`
+    // diagram (rendered App-side) can choose side-by-side vs vertical.
+    app.last_output_width = inner.width;
 
     let lines: Vec<Line<'_>> = app
         .output
@@ -756,6 +759,19 @@ const fn output_span_style(class: OutputStyleClass, theme: &Theme) -> Style {
         // existing `client_side.*` notes). `theme.muted` is the
         // established dim foreground.
         OutputStyleClass::Hint => Style::new().fg(theme.muted),
+        // ADR-0044 relationship diagrams. Reuse existing theme colours
+        // (no new Theme fields): the table name stands out via weight,
+        // keys + cardinality take accent colours, connectors are muted.
+        OutputStyleClass::DiagramTableName => {
+            Style::new().fg(theme.fg).add_modifier(Modifier::BOLD)
+        }
+        OutputStyleClass::DiagramKey => Style::new()
+            .fg(theme.plan_efficient)
+            .add_modifier(Modifier::BOLD),
+        OutputStyleClass::DiagramCardinality => Style::new()
+            .fg(theme.tok_number)
+            .add_modifier(Modifier::BOLD),
+        OutputStyleClass::DiagramConnector => Style::new().fg(theme.muted),
     }
 }