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.

src/app.rs

@@ -2110,8 +2110,10 @@ impl App {
             // 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.
+            // relationships as compact diagrams. Every other (incidental
+            // DDL) echo renders structure only — no relationship block
+            // at all (ADR-0050, issue #28; supersedes ADR-0044 §1's
+            // prose retention for these surfaces).
             if matches!(
                 command,
                 Command::ShowTable { .. }