M7: area type vertical alignment

Area Type Options > Align: Top / Center / Bottom / Justify (AreaOptions.verticalAlign,
LayoutOptions.vertical_align). Each row/column cell is aligned on its own; rectangular
cells shift their lines, other frames and text wrap reflow until settled without
losing text. Justify shares the leftover space equally between line gaps. Area Type Options
shows Align and First Baseline as dropdowns (the parameter dialog's choices table).

crates/format/tests/native_v3.rs

@@ -43,6 +43,31 @@ fn opts(version: u32, compress: bool) -> SaveOptions {
     SaveOptions { version, compress, ..SaveOptions::default() }
 }
 
+#[test]
+fn area_type_vertical_alignment_round_trips() {
+    let d = rich_doc();
+    let text = String::from_utf8(save(&d, false)).unwrap();
+    assert!(text.contains("\"verticalAlign\":\"center\""), "the fixture's area type is centred");
+    let back = load(text.as_bytes()).unwrap();
+    assert_eq!(doc_json(&back), doc_json(&d));
+    let area = |d: &Document| {
+        let mut found = None;
+        d.walk(|n| {
+            if let NodeKind::Text(t) = &n.kind
+                && matches!(t.kind, vectorcraft_doc::TextKind::Area { .. })
+            {
+                found = Some(t.area.vertical_align);
+            }
+        });
+        found.unwrap()
+    };
+    assert_eq!(area(&back), vectorcraft_doc::VerticalAlign::Center);
+    // Files from before the option open top-aligned.
+    let old = text.replacen("\"verticalAlign\":\"center\",", "", 1).replacen(",\"verticalAlign\":\"center\"", "", 1);
+    assert!(!old.contains("verticalAlign"));
+    assert_eq!(area(&load(old.as_bytes()).unwrap()), vectorcraft_doc::VerticalAlign::Top);
+}
+
 #[test]
 fn compressed_files_load() {
     let d = rich_doc();