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/event.rs

@@ -73,6 +73,9 @@ pub enum AppEvent {
     /// An `explain …` command succeeded (ADR-0028). `plan`
     /// carries the captured query plan; nothing was executed.
     DslExplainSucceeded { command: Command, plan: QueryPlan },
+    /// A `show <kind>` list command (V5) — carries pre-formatted
+    /// display lines (tables / relationships / indexes).
+    DslShowListSucceeded { command: Command, lines: Vec<String> },
     DslInsertSucceeded {
         command: Command,
         result: InsertResult,