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/event.rs

@@ -9,7 +9,8 @@ use crossterm::event::KeyEvent;
 
 use crate::db::{
     AddColumnResult, ChangeColumnTypeResult, DataResult, DbError, DeleteResult,
-    DropColumnResult, InsertResult, QueryPlan, TableDescription, UpdateResult,
+    DropColumnResult, InsertResult, QueryPlan, RelationshipDiagramData, TableDescription,
+    UpdateResult,
 };
 use crate::dsl::Command;
 
@@ -76,6 +77,12 @@ pub enum AppEvent {
     /// A `show <kind>` list command (V5) — carries pre-formatted
     /// display lines (tables / relationships / indexes).
     DslShowListSucceeded { command: Command, lines: Vec<String> },
+    /// `show relationship <name>` (ADR-0044) — structured data for the
+    /// diagram, rendered App-side; `None` when no such relationship.
+    DslShowRelationshipSucceeded {
+        command: Command,
+        data: Option<RelationshipDiagramData>,
+    },
     DslInsertSucceeded {
         command: Command,
         result: InsertResult,