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/walking_skeleton.rs

@@ -473,9 +473,15 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
         echo: None,
     });
 
-    let rendered = rendered_text(&mut app, &Theme::dark(), 80, 24);
-    assert!(rendered.contains("Referenced by:"), "{rendered}");
-    assert!(rendered.contains("Orders.CustId"), "{rendered}");
+    // Tall viewport so the [ok] echo line stays visible above the
+    // (taller-than-prose) diagram for the endpoint-subject assertion.
+    let rendered = rendered_text(&mut app, &Theme::dark(), 80, 40);
+    // ADR-0044: `add relationship` is relationship-relevant, so its echo
+    // renders the relationship as a compact diagram, not the prose block.
+    assert!(rendered.contains("Relationships"), "heading: {rendered}");
+    assert!(rendered.contains("Orders"), "neighbour box: {rendered}");
+    assert!(rendered.contains("CustId"), "FK column: {rendered}");
+    assert!(rendered.contains('▶'), "connector: {rendered}");
     assert!(rendered.contains("on delete cascade"), "{rendered}");
     // The [ok] subject lists the endpoints. Long lines wrap in
     // the panel, so we check the first half of the phrase only.