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/completion.rs

@@ -1661,9 +1661,21 @@ mod tests {
     }
 
     #[test]
-    fn show_offers_data_and_table_alphabetised() {
+    fn show_offers_all_subcommands() {
+        // `show` branches: data / table (singular) plus the V5
+        // list-all forms tables / relationships / indexes, in
+        // grammar-declaration order.
         let cs = cands("show ", 5);
-        assert_eq!(cs, vec!["data".to_string(), "table".to_string()]);
+        assert_eq!(
+            cs,
+            vec![
+                "data".to_string(),
+                "table".to_string(),
+                "tables".to_string(),
+                "relationships".to_string(),
+                "indexes".to_string(),
+            ],
+        );
     }
 
     #[test]