feat(render): incidental-DDL confirmations show structure only, no relationships (#28)

Per ADR-0050 (closing issue #28): the confirmation echo after an
incidental structural edit — create table, add/drop/rename/change
column, add/drop index — now renders the structure only (header +
column box + indexes + constraints) and no longer appends the
References:/Referenced by: relationship block.

Rationale: a confirmation reports the change just made, not the
table's relationships, which the user didn't touch. Relationship info
is still one `show table <T>` away, and the relationship-subject
surfaces (show table, add/drop relationship) keep their ADR-0044
diagrams unchanged.

Scope is all incidental DDL (user-confirmed). Mechanism: drop the
relationship-block call from render_structure (all its callers are
incidental DDL); the handle_dsl_success diagram-vs-structure routing
is unchanged. The orphaned relationship_prose_lines + cols_disp
helpers are deleted (the prose format survives in ADR-0016 §5 + git
history for a future OOS-7 always-prose setting).

ADR-0050 supersedes ADR-0044 §1's incidental-DDL prose clause and the
relationship-block half of ADR-0016 §5 (both annotated). Tests: prose-
presence unit test + snapshot removed; new unit test locks structure-
only with inbound+outbound relationships present; the misnamed add-
column integration test inverted + renamed. 2458 pass / 0 fail / 0
skip, clippy clean.

tests/it/walking_skeleton.rs

@@ -494,7 +494,12 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
 }
 
 #[test]
-fn add_relationship_flow_shows_inbound_section_on_parent() {
+fn add_column_confirmation_omits_relationship_prose() {
+    // ADR-0050 (issue #28): an incidental-DDL confirmation echo (here
+    // `add column`) renders the structure only — never the
+    // `References:` / `Referenced by:` relationship block — even when
+    // the table carries relationships the user did not touch. The
+    // relationships remain one `show table` away.
     let mut app = App::new();
     let customers = TableDescription {
         name: "Customers".to_string(),
@@ -535,8 +540,17 @@ fn add_relationship_flow_shows_inbound_section_on_parent() {
         echo: None,
     });
     let rendered = rendered_text(&mut app, &Theme::dark(), 80, 24);
-    assert!(rendered.contains("Referenced by:"), "{rendered}");
-    assert!(rendered.contains("Orders.CustId → Id"), "{rendered}");
+    // The structure box still renders (table name + the column box from
+    // the returned description).
+    assert!(rendered.contains("Customers"), "structure header:\n{rendered}");
+    assert!(rendered.contains("Constraints"), "structure box:\n{rendered}");
+    // The relationship block is gone — neither prose heading nor line.
+    assert!(!rendered.contains("Referenced by:"), "no prose heading:\n{rendered}");
+    assert!(!rendered.contains("References:"), "no prose heading:\n{rendered}");
+    assert!(
+        !rendered.contains("Orders.CustId → Id"),
+        "no prose line:\n{rendered}",
+    );
 }
 
 #[test]