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).

src/app.rs

@@ -1689,8 +1689,28 @@ impl App {
     fn handle_dsl_success(&mut self, command: &Command, description: Option<TableDescription>) {
         self.note_ok_summary(command);
         if let Some(desc) = description.as_ref() {
-            for line in crate::output_render::render_structure(desc) {
-                self.note_system(line);
+            // ADR-0044 §1 "relationship-relevant" reach: when a
+            // relationship is the subject of the command (`show table`,
+            // `add`/`drop relationship`), render the table's
+            // relationships as compact diagrams; every other DDL echo
+            // keeps the prose `References:` / `Referenced by:` form.
+            if matches!(
+                command,
+                Command::ShowTable { .. }
+                    | Command::AddRelationship { .. }
+                    | Command::DropRelationship { .. }
+            ) {
+                for line in crate::output_render::render_structure_with_diagrams(
+                    desc,
+                    self.last_output_width,
+                    self.mode,
+                ) {
+                    self.push_output(line);
+                }
+            } else {
+                for line in crate::output_render::render_structure(desc) {
+                    self.note_system(line);
+                }
             }
         }
         self.current_table = description;