Print structured refs to columns starting with @ in double brackets

Found while testing crates/formula/src/printer.rs:164 (write_struct_ref, single column without specifiers)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/formula/src/printer.rs

@@ -175,8 +175,8 @@ fn write_struct_ref(s: &StructRef, at_shorthand: bool) -> String {
             let _ = write!(out, "[{}]", s.specifiers.first().map(item).unwrap_or(""));
         }
         (Some(c), None) if s.specifiers.is_empty() => {
-            // Excel writes a name with spaces at its edges in the double-bracket form.
-            if c.trim() == c {
+            // Names with spaces at their edges, or starting with `@` (this-row shorthand), use the double-bracket form.
+            if c.trim() == c && !c.starts_with('@') {
                 let _ = write!(out, "[{}]", esc(c));
             } else {
                 let _ = write!(out, "[[{}]]", esc(c));

crates/formula/src/tests.rs

@@ -558,3 +558,12 @@ fn reference_shapes_print_unchanged() -> Result<(), Box<dyn std::error::Error>>
     }
     Ok(())
 }
+
+#[test]
+fn column_starting_with_at_round_trips() {
+    for name in ["@a", "a@b"] {
+        let s = StructRef { table: "T".into(), specifiers: vec![], col_start: Some(name.into()), col_end: None };
+        let text = format!("={}", printer::struct_ref(&s));
+        assert_eq!(parse(&text).ok(), Some(Expr::Struct(s)), "printed as {text}");
+    }
+}