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/it/parse_error_pedagogy.rs

@@ -168,6 +168,8 @@ fn near_miss_matrix_committed_multiforms() {
         ("drop index on T", false, &["after `drop index on T`, expected `(`", "drop index on <Table>"]),
         ("drop relationship", false, &["after `drop relationship`, expected `from` or relationship name", "drop relationship <Name>"]),
         ("show table", false, &["after `show table`, expected table name", "show table <Table>"]),
+        ("show relationship", false, &["after `show relationship`, expected relationship name", "show relationship <name>"]),
+        ("show index", false, &["after `show index`, expected index name", "show index <name>"]),
         ("change column in table T: c", false, &["after `change column in table T: c`, expected `(`", "change column [in] [table]"]),
         // advanced committed multi-forms
         ("create index on", true, &["after `create index on`, expected table name", "create [unique] index"]),