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.

docs/requirements.md

@@ -450,13 +450,29 @@ since ADR-0027.)
   buffer is in, with new output snapping the view to the most
   recent. The full V4 scope — smart structure rendering, log
   styling, Markdown export, scroll indicator — remains pending.)*
-- [/] **V5** `show <kind> [<name>]` family of commands for
+- [x] **V5** `show <kind> [<name>]` family of commands for
   redisplaying schema info on demand.
-  *(Partial, verified 2026-06-07: `show table <name>` and
-  `show data <Table>` implemented (`grammar/data.rs`). **Missing
-  the "all items" variants** — `show tables`, `show
-  relationships`, `show indexes` — none are registered. V5 closes
-  when the plural/list forms land.)*
+  *(Done 2026-06-07: `show table <name>` + `show data <Table>`
+  (single-item) plus the list-all family `show tables` /
+  `show relationships` / `show indexes` — the latter three landed
+  as `Command::ShowList { kind }` (one variant, `grammar/data.rs`),
+  a read-only worker `show_list` formatting count-headed lists from
+  the same helpers the items panel uses, with help + parse-usage
+  entries and 10 integration tests (`tests/it/show_list.rs`). The
+  one remaining member of the `[<name>]` clause — singular
+  per-item detail for relationships/indexes — is split out as
+  **V5a** below so it has a tracked home rather than living as a
+  footnote here.)*
+- [ ] **V5a** Singular per-item detail views `show relationship
+  <name>` / `show index <name>` — the `[<name>]` half of V5 for
+  the relationship and index kinds (the table kind already has
+  `show table <name>`). Each names one item and renders its
+  detail (a relationship's endpoints + ON DELETE/UPDATE actions; an
+  index's table, columns, and uniqueness). Small follow-up on the
+  V5 machinery: a name slot after the `relationship` / `index`
+  keyword in `SHOW_CHOICES`, a lookup-one in the worker, and a
+  detail render. Not yet built; raised 2026-06-07 when V5's
+  list-all family shipped.
 - [x] **V6** Copy the output panel to the system clipboard
   (issue #11, ADR-0041). `copy` / `copy all` copy the whole
   panel; `copy last` copies the most recent command's output.