feat: V5a show relationship/index <name> detail views

Fold the singular per-item forms into Command::ShowList { kind,
name: Option<String> } (name: Some = one item). Two grammar
branches reuse the relationship/index completion sources; worker
do_show_one renders a labelled detail block or a friendly
"No ... named X." line, reusing the V5 render path. Help +
parse-usage entries, two ADR-0042 near-miss rows, 5 integration
tests. Mark V5a [x] — V5's [<name>] clause now complete.

src/runtime.rs

@@ -2774,9 +2774,10 @@ async fn execute_command_typed(
             .describe_table(name, src)
             .await
             .map(|d| CommandOutcome::Schema(Some(d))),
-        Command::ShowList { kind } => {
-            database.show_list(kind).await.map(CommandOutcome::ShowList)
-        }
+        Command::ShowList { kind, name } => database
+            .show_list(kind, name)
+            .await
+            .map(CommandOutcome::ShowList),
         Command::Insert {
             table,
             columns,