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.

tests/typing_surface/mod.rs

@@ -233,6 +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:?})"),
         Insert { .. } => "Insert".into(),
         Update { .. } => "Update".into(),
         Delete { .. } => "Delete".into(),