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.

tests/typing_surface/mod.rs

@@ -233,7 +233,7 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         AddConstraint { .. } => "AddConstraint".into(),
         DropConstraint { .. } => "DropConstraint".into(),
         ShowTable { .. } => "ShowTable".into(),
-        ShowList { kind } => format!("ShowList({kind:?})"),
+        ShowList { kind, name } => format!("ShowList({kind:?}, {})", name.is_some()),
         Insert { .. } => "Insert".into(),
         Update { .. } => "Update".into(),
         Delete { .. } => "Delete".into(),