feat: V5 show tables / relationships / indexes list commands

Add the list-all show family as one Command::ShowList { kind }
variant. A read-only worker show_list formats count-headed lists
(reusing do_list_tables / read_all_relationships /
read_table_indexes, so it never drifts from the items panel);
internal __rdbms_* tables excluded. Help + parse-usage entries
added; 10 integration tests in tests/it/show_list.rs.

Mark V5 [x]. Split the singular show relationship/index <name>
detail forms (the [<name>] half) into a new tracked V5a [ ] item
rather than leaving them as an untracked footnote.

src/friendly/keys.rs

@@ -306,6 +306,9 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.select", &[]),
     ("parse.usage.show_data", &[]),
     ("parse.usage.show_table", &[]),
+    ("parse.usage.show_tables", &[]),
+    ("parse.usage.show_relationships", &[]),
+    ("parse.usage.show_indexes", &[]),
     ("parse.usage.update", &[]),
     ("parse.usage.with", &[]),
     ("parse.expect.select_projection", &[]),