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.

src/dsl/command.rs

@@ -332,12 +332,16 @@ pub enum Command {
     ShowTable {
         name: String,
     },
-    /// Re-display a whole schema collection — every table,
-    /// relationship, or index — as a list in the output (V5). The
-    /// read-only "all items" sibling of `ShowTable`; pure display,
-    /// no schema change.
+    /// Re-display a schema collection in the output (V5/V5a). With
+    /// `name: None`, the whole collection — every table,
+    /// relationship, or index (the list-all forms). With
+    /// `name: Some(_)`, one named item's detail — `show
+    /// relationship <name>` / `show index <name>` (V5a); the table
+    /// singular is the separate `ShowTable`, so a named `Tables`
+    /// never occurs. Read-only; pure display, no schema change.
     ShowList {
         kind: ShowListKind,
+        name: Option<String>,
     },
     /// Insert a single row. `columns` is `None` for the natural-
     /// order short form (`insert into T values (...)`); the
@@ -903,7 +907,17 @@ impl Command {
             Self::AddConstraint { .. } => "add constraint",
             Self::DropConstraint { .. } => "drop constraint",
             Self::ShowTable { .. } => "show table",
-            Self::ShowList { kind } => kind.command_name(),
+            // A named item reports the singular verb; the list-all
+            // forms report the plural (`kind.command_name()`).
+            Self::ShowList {
+                kind: ShowListKind::Relationships,
+                name: Some(_),
+            } => "show relationship",
+            Self::ShowList {
+                kind: ShowListKind::Indexes,
+                name: Some(_),
+            } => "show index",
+            Self::ShowList { kind, .. } => kind.command_name(),
             Self::Insert { .. } => "insert into",
             Self::Update { .. } => "update",
             Self::Delete { .. } => "delete from",