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/grammar/data.rs

@@ -107,12 +107,53 @@ const SHOW_TABLES: Node = Node::Word(Word::keyword("tables"));
 const SHOW_RELATIONSHIPS: Node = Node::Word(Word::keyword("relationships"));
 const SHOW_INDEXES: Node = Node::Word(Word::keyword("indexes"));
 
+// `show relationship <name>` / `show index <name>` — singular
+// per-item detail (V5a). The name slot reuses the existing
+// completion sources (relationship / index names). Distinct
+// keyword tokens from the plurals (`relationship` ≠
+// `relationships`), so Choice ordering is irrelevant.
+const SHOW_RELATIONSHIP_NAME: Node = Node::Ident {
+    source: IdentSource::Relationships,
+    role: "relationship_name",
+    validator: None,
+    highlight_override: None,
+    writes_table: false,
+    writes_column: false,
+    writes_user_listed_column: false,
+    writes_table_alias: false,
+    writes_cte_name: false,
+    writes_projection_alias: false,
+};
+const SHOW_RELATIONSHIP_NODES: &[Node] = &[
+    Node::Word(Word::keyword("relationship")),
+    SHOW_RELATIONSHIP_NAME,
+];
+const SHOW_RELATIONSHIP: Node = Node::Seq(SHOW_RELATIONSHIP_NODES);
+
+const SHOW_INDEX_NAME: Node = Node::Ident {
+    source: IdentSource::Indexes,
+    role: "index_name",
+    validator: None,
+    highlight_override: None,
+    writes_table: false,
+    writes_column: false,
+    writes_user_listed_column: false,
+    writes_table_alias: false,
+    writes_cte_name: false,
+    writes_projection_alias: false,
+};
+const SHOW_INDEX_NODES: &[Node] =
+    &[Node::Word(Word::keyword("index")), SHOW_INDEX_NAME];
+const SHOW_INDEX: Node = Node::Seq(SHOW_INDEX_NODES);
+
 const SHOW_CHOICES: &[Node] = &[
     SHOW_DATA,
     SHOW_TABLE,
     SHOW_TABLES,
     SHOW_RELATIONSHIPS,
     SHOW_INDEXES,
+    SHOW_RELATIONSHIP,
+    SHOW_INDEX,
 ];
 const SHOW_SHAPE: Node = Node::Choice(SHOW_CHOICES);
 
@@ -576,12 +617,24 @@ fn build_show(path: &MatchedPath, _source: &str) -> Result<Command, ValidationEr
         }),
         Some("tables") => Ok(Command::ShowList {
             kind: ShowListKind::Tables,
+            name: None,
         }),
         Some("relationships") => Ok(Command::ShowList {
             kind: ShowListKind::Relationships,
+            name: None,
         }),
         Some("indexes") => Ok(Command::ShowList {
             kind: ShowListKind::Indexes,
+            name: None,
+        }),
+        // V5a singular per-item detail — carry the named item.
+        Some("relationship") => Ok(Command::ShowList {
+            kind: ShowListKind::Relationships,
+            name: Some(require_ident(path, "relationship_name")?),
+        }),
+        Some("index") => Ok(Command::ShowList {
+            kind: ShowListKind::Indexes,
+            name: Some(require_ident(path, "index_name")?),
         }),
         _ => Err(ValidationError {
             message_key: "parse.error_wrapper",
@@ -1395,6 +1448,8 @@ pub static SHOW: CommandNode = CommandNode {
         "parse.usage.show_tables",
         "parse.usage.show_relationships",
         "parse.usage.show_indexes",
+        "parse.usage.show_relationship",
+        "parse.usage.show_index",
     ],};
 
 pub static INSERT: CommandNode = CommandNode {