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/runtime.rs

@@ -1407,6 +1407,12 @@ fn spawn_dsl_dispatch(
                 command: command.clone(),
                 lines,
             },
+            Ok(CommandOutcome::ShowRelationship(data)) => {
+                AppEvent::DslShowRelationshipSucceeded {
+                    command: command.clone(),
+                    data: data.map(|b| *b),
+                }
+            }
             Ok(CommandOutcome::QueryPlan(plan)) => AppEvent::DslExplainSucceeded {
                 command: command.clone(),
                 plan,
@@ -2252,6 +2258,10 @@ enum CommandOutcome {
     /// the worker (table / relationship / index names). Pure
     /// display, no schema change.
     ShowList(Vec<String>),
+    /// Structured data for one relationship's diagram (ADR-0044),
+    /// rendered App-side; `None` when the named relationship is absent.
+    /// Boxed — two full `TableDescription`s dwarf the other variants.
+    ShowRelationship(Option<Box<crate::db::RelationshipDiagramData>>),
     QueryPlan(QueryPlan),
     Insert(InsertResult),
     Update(UpdateResult),
@@ -2774,6 +2784,16 @@ async fn execute_command_typed(
             .describe_table(name, src)
             .await
             .map(|d| CommandOutcome::Schema(Some(d))),
+        // ADR-0044: a named relationship renders as a diagram (App-side),
+        // so it returns structured data; every other `show <kind>` form
+        // stays the worker-formatted prose list.
+        Command::ShowList {
+            kind: crate::dsl::command::ShowListKind::Relationships,
+            name: Some(name),
+        } => database
+            .show_relationship(name)
+            .await
+            .map(|opt| CommandOutcome::ShowRelationship(opt.map(Box::new))),
         Command::ShowList { kind, name } => database
             .show_list(kind, name)
             .await