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/friendly/keys.rs

@@ -311,6 +311,8 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.show_tables", &[]),
     ("parse.usage.show_relationships", &[]),
     ("parse.usage.show_indexes", &[]),
+    ("parse.usage.show_relationship", &[]),
+    ("parse.usage.show_index", &[]),
     ("parse.usage.update", &[]),
     ("parse.usage.with", &[]),
     ("parse.expect.select_projection", &[]),