feat: V5a show relationship/index <name> detail views

Fold the singular per-item forms into Command::ShowList { kind,
name: Option<String> } (name: Some = one item). Two grammar
branches reuse the relationship/index completion sources; worker
do_show_one renders a labelled detail block or a friendly
"No ... named X." line, reusing the V5 render path. Help +
parse-usage entries, two ADR-0042 near-miss rows, 5 integration
tests. Mark V5a [x] — V5's [<name>] clause now complete.

docs/requirements.md

@@ -463,16 +463,21 @@ since ADR-0027.)
   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
+- [x] **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.
+  `show table <name>`).
+  *(Done 2026-06-07: folded into `Command::ShowList { kind, name:
+  Option<String> }` — `name: Some(_)` is the singular form. Two
+  grammar branches (`relationship <name>` / `index <name>`,
+  reusing the `Relationships`/`Indexes` completion sources for the
+  name slot), a worker `do_show_one` rendering a labelled detail
+  block (endpoints + ON DELETE/UPDATE for a relationship; table,
+  columns, uniqueness for an index) or a friendly "No relationship/
+  index named `X`." line, reusing the V5 `ShowList` render path.
+  Help + parse-usage entries + two ADR-0042 near-miss matrix rows;
+  5 added integration tests. V5's `[<name>]` clause is now
+  complete across all three kinds.)*
 - [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.