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/app.rs

@@ -76,6 +76,19 @@ pub enum OutputStyleClass {
     /// every `[client-side]` category-3 prose note (ADR-0038 §6).
     /// Resolves to `theme.muted`.
     Hint,
+    /// A relationship-diagram box's title row — the table name
+    /// (ADR-0044 §2.1). Bold accent so it cannot read as a column.
+    DiagramTableName,
+    /// A relationship-diagram key marker — `(PK)` / `●` on the
+    /// participating columns (ADR-0044 §2.2).
+    DiagramKey,
+    /// A relationship-diagram cardinality label — `1` / `n`
+    /// (ADR-0044 §2).
+    DiagramCardinality,
+    /// A relationship-diagram connector — box-drawing line, elbows
+    /// and arrowhead between the two boxes (ADR-0044 §2.3). Muted so
+    /// the structure, not the wiring, leads.
+    DiagramConnector,
 }
 
 /// A styled span of an output line: a byte range over the
@@ -268,6 +281,11 @@ pub struct App {
     /// logical OutputLines. Required for accurate scroll capping
     /// when long lines wrap to multiple display rows.
     pub last_output_total_wrapped: usize,
+    /// The most recent inner width (in columns) of the output panel,
+    /// recorded by the renderer (ADR-0044 §3). Drives the relationship
+    /// diagram's side-by-side vs vertical layout choice. Defaults to
+    /// `80` until the first render measures the real width.
+    pub last_output_width: u16,
     /// Prettified display name of the currently-open project,
     /// rendered in the status bar (P-NAME-3, ADR-0015 §2). `None`
     /// during very-early startup before the runtime has opened a
@@ -432,6 +450,7 @@ impl App {
             output_scroll: 0,
             last_output_visible: 0,
             last_output_total_wrapped: 0,
+            last_output_width: 80,
             project_name: None,
             project_is_temp: false,
             fatal_message: None,
@@ -614,6 +633,10 @@ impl App {
                 }
                 Vec::new()
             }
+            AppEvent::DslShowRelationshipSucceeded { command, data } => {
+                self.handle_dsl_show_relationship_success(&command, data.as_ref());
+                Vec::new()
+            }
             AppEvent::DslInsertSucceeded { command, result } => {
                 self.handle_dsl_insert_success(&command, &result);
                 Vec::new()
@@ -1694,6 +1717,35 @@ impl App {
         }
     }
 
+    /// `show relationship <name>` (ADR-0044): render the relationship
+    /// as a styled two-table diagram, App-side, sized to the current
+    /// output-panel width. `None` is the friendly not-found line.
+    fn handle_dsl_show_relationship_success(
+        &mut self,
+        command: &Command,
+        data: Option<&crate::db::RelationshipDiagramData>,
+    ) {
+        self.note_ok_summary(command);
+        match data {
+            Some(data) => {
+                for line in crate::output_render::render_relationship_diagram(
+                    data,
+                    self.last_output_width,
+                    self.mode,
+                ) {
+                    self.push_output(line);
+                }
+            }
+            None => {
+                let name = match command {
+                    Command::ShowList { name: Some(n), .. } => n.as_str(),
+                    _ => "",
+                };
+                self.note_system(format!("No relationship named `{name}`."));
+            }
+        }
+    }
+
     fn handle_dsl_insert_success(&mut self, command: &Command, result: &InsertResult) {
         self.note_ok_summary(command);
         self.note_system(crate::t!("ok.rows_inserted", count = result.rows_affected));