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/db.rs

@@ -80,6 +80,21 @@ pub struct TableDescription {
     pub check_constraints: Vec<crate::persistence::TableCheck>,
 }
 
+/// Structured payload for rendering one relationship's diagram.
+///
+/// ADR-0044: the relationship plus both endpoint table structures.
+/// Built worker-side; rendered **App-side** (like `QueryPlan`) so the
+/// diagram can be width-aware and styled.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RelationshipDiagramData {
+    /// The relationship itself (endpoints + referential actions).
+    pub rel: crate::persistence::RelationshipSchema,
+    /// FK-holder (the `n` side), drawn on the left.
+    pub child: TableDescription,
+    /// Referenced table (the `1` side), drawn on the right.
+    pub parent: TableDescription,
+}
+
 /// One user-created index on a table (ADR-0025).
 ///
 /// Read live from the engine's native catalog
@@ -566,6 +581,13 @@ enum Request {
         name: Option<String>,
         reply: oneshot::Sender<Result<Vec<String>, DbError>>,
     },
+    /// Structured data to render one relationship's diagram (ADR-0044
+    /// §6): the relationship + both endpoint table structures, or
+    /// `None` if no relationship by that name exists.
+    ShowRelationship {
+        name: String,
+        reply: oneshot::Sender<Result<Option<RelationshipDiagramData>, DbError>>,
+    },
     DescribeTable {
         name: String,
         source: Option<String>,
@@ -1341,6 +1363,18 @@ impl Database {
         recv.await.map_err(|_| DbError::WorkerGone)?
     }
 
+    /// Structured data to render one relationship's diagram (ADR-0044):
+    /// the relationship + both endpoint table structures, or `None` if
+    /// no relationship by that name exists.
+    pub async fn show_relationship(
+        &self,
+        name: String,
+    ) -> Result<Option<RelationshipDiagramData>, DbError> {
+        let (reply, recv) = oneshot::channel();
+        self.send(Request::ShowRelationship { name, reply }).await?;
+        recv.await.map_err(|_| DbError::WorkerGone)?
+    }
+
     pub async fn describe_table(
         &self,
         name: String,
@@ -2272,6 +2306,9 @@ fn handle_request(
         Request::ShowList { kind, name, reply } => {
             let _ = reply.send(do_show_list(conn, kind, name.as_deref()));
         }
+        Request::ShowRelationship { name, reply } => {
+            let _ = reply.send(do_show_relationship(conn, &name));
+        }
         Request::DescribeTable {
             name,
             source,
@@ -5870,6 +5907,25 @@ fn do_list_tables(conn: &Connection) -> Result<Vec<String>, DbError> {
     Ok(out)
 }
 
+/// Structured data to render one relationship's diagram (ADR-0044):
+/// find the named relationship, then describe both endpoint tables.
+/// `Ok(None)` when no relationship by that name exists (the App shows
+/// a friendly not-found line).
+fn do_show_relationship(
+    conn: &Connection,
+    name: &str,
+) -> Result<Option<RelationshipDiagramData>, DbError> {
+    let Some(rel) = read_all_relationships(conn)?
+        .into_iter()
+        .find(|r| r.name == name)
+    else {
+        return Ok(None);
+    };
+    let child = do_describe_table(conn, &rel.child_table)?;
+    let parent = do_describe_table(conn, &rel.parent_table)?;
+    Ok(Some(RelationshipDiagramData { rel, child, parent }))
+}
+
 /// Pre-formatted display lines for the `show <kind>` list commands
 /// (V5). A count header followed by one indented item per line, or a
 /// single friendly "none yet" line for an empty collection. Reuses