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/dsl/mod.rs

@@ -23,7 +23,7 @@ pub use action::ReferentialAction;
 pub use command::{
     AlterTableAction, AppCommand, ChangeColumnMode, ColumnSpec, Command, CompareOp, CopyScope, Expr,
     IndexSelector, MessagesValue, ModeValue, Operand, Predicate, RelationshipSelector, RowFilter,
-    SqlForeignKey,
+    ShowListKind, SqlForeignKey,
 };
 pub use parser::{ParseError, parse_command};
 pub use types::Type;