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/app.rs

@@ -2890,6 +2890,8 @@ mod tests {
             "show tables",
             "show relationships",
             "show indexes",
+            "show relationship",
+            "show index",
         ] {
             app.update(key(KeyCode::Tab));
             assert_eq!(app.input, expected);
@@ -2903,8 +2905,9 @@ mod tests {
     fn shift_tab_cycles_backward_starting_from_last() {
         let mut app = App::new();
         type_str(&mut app, "show ");
-        // Backward starts from the last candidate (`indexes`).
-        for expected in ["show indexes", "show relationships", "show tables"] {
+        // Backward starts from the last candidate (`index`, the
+        // V5a singular form).
+        for expected in ["show index", "show relationship", "show indexes"] {
             app.update(key(KeyCode::BackTab));
             assert_eq!(app.input, expected);
         }