feat: show table renders relationships as compact diagrams (ADR-0044)

show table <T> and add/drop relationship echoes now render the focal
structure box plus a Relationships section of compact stacked
connector diagrams (child-left/parent-right, n…1, actions); incidental
DDL echoes keep the prose References:/Referenced by: form. Selected by
command in handle_dsl_success via the "relationship-relevant" reach.

- output_render.rs: render_structure refactored into section helpers
  (box/prose/index/constraint), byte-identical output; new
  render_structure_with_diagrams + compact-box rendering
- app.rs: handle_dsl_success routes ShowTable/Add/DropRelationship to
  the diagram path, others to prose
- fixes: eager widths[1] index on compact (1-col) boxes; body-cell
  padding under title-widening (name wider than columns)

Tests: unit + snapshot + integration; add-relationship echo test
updated to the diagram form. Full suite 2203 pass / 0 fail / 1 ignored;
clippy clean. V1 still [/] (compound routing + self-ref remain).

tests/it/show_list.rs

@@ -454,3 +454,38 @@ fn app_show_relationship_not_found_shows_friendly_line() {
         "friendly not-found line",
     );
 }
+
+#[test]
+fn app_show_table_renders_relationships_as_compact_diagrams() {
+    let (_p, db, _dir) = open_project_db();
+    let rt = rt();
+    rt.block_on(seed_schema(&db));
+    // Orders holds the FK to Customers — an outbound relationship.
+    let desc = rt
+        .block_on(db.describe_table("Orders".to_string(), None))
+        .expect("describe Orders");
+
+    let mut app = App::new();
+    app.output.push_back(rdbms_playground::app::OutputLine::echo(
+        "show table Orders",
+        Mode::Simple,
+    ));
+    app.update(AppEvent::DslSucceeded {
+        command: Command::ShowTable {
+            name: "Orders".to_string(),
+        },
+        description: Some(desc),
+        echo: None,
+    });
+    let text: String = app
+        .output
+        .iter()
+        .map(|l| l.text.as_str())
+        .collect::<Vec<_>>()
+        .join("\n");
+    // The focal structure box, then a diagram (not the prose block).
+    assert!(text.contains("Relationships"), "diagram heading: {text}");
+    assert!(!text.contains("References:"), "prose suppressed: {text}");
+    assert!(text.contains("Customers"), "neighbour box: {text}");
+    assert!(text.contains('▶'), "connector arrow: {text}");
+}