M10: Flutter code writes no spacing beside a space distribution

flutter_line skipped `spacing:` when the main-axis alignment string
contained "Space", but the strings are MainAxisAlignment.spaceBetween,
spaceAround and spaceEvenly (lower-case s), so it never matched and the
gap was written on top of the distribution. It now checks the MainAlign
value itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Model-Effort: Opus 5.5 drove and wrote; Fable 5.2 spotted it

crates/engine/src/dev/codegen.rs

@@ -675,6 +675,27 @@ mod tests {
         assert!(!dart.lines().any(|l| l.trim_start().starts_with("void main()")), "a name never leaves its comment: {dart}");
     }
 
+    #[test]
+    fn flutter_writes_no_spacing_for_the_space_distributions() {
+        // The space distributions spread the children themselves; a `spacing:` beside them would
+        // add the gap on top.
+        let mut s = Session::new();
+        let row = id_of(&s.execute("frame.create", &json!({"x": 0, "y": 0, "width": 300, "height": 40})).unwrap()["id"]).unwrap();
+        s.execute("shape.rectangle", &json!({"x": 0, "y": 0, "width": 50, "height": 20, "parent": row})).unwrap();
+        s.execute("shape.rectangle", &json!({"x": 60, "y": 0, "width": 50, "height": 20, "parent": row})).unwrap();
+        s.execute("layout.add", &json!({"ids": [row], "direction": "horizontal"})).unwrap();
+        for (align, flutter) in [("spaceBetween", "spaceBetween"), ("spaceAround", "spaceAround"), ("spaceEvenly", "spaceEvenly")] {
+            s.execute("node.set", &json!({"id": row, "props": {"gap": 10, "mainAlign": align}})).unwrap();
+            let dart = code_for(s.doc().unwrap(), row, CodeLang::Flutter);
+            assert!(dart.contains(&format!("mainAxisAlignment: MainAxisAlignment.{flutter},")), "{dart}");
+            assert!(!dart.contains("spacing:"), "{align} spreads the children itself: {dart}");
+        }
+        // A packed line keeps its gap.
+        s.execute("node.set", &json!({"id": row, "props": {"gap": 10, "mainAlign": "center"}})).unwrap();
+        let dart = code_for(s.doc().unwrap(), row, CodeLang::Flutter);
+        assert!(dart.contains("spacing: 10,"), "{dart}");
+    }
+
     #[test]
     fn flutter_expands_and_stretches_fill_children() {
         let mut s = Session::new();

crates/engine/src/dev/codegen/flutter.rs

@@ -335,7 +335,7 @@ fn flutter_line(ctx: &Ctx, l: &AutoLayout, flow: &[Placed], slot: Option<&Param>
         out.push_str(&format!("{inner}textBaseline: TextBaseline.alphabetic,\n"));
     }
     // The gap between the children; the space distributions already spread them.
-    if l.gap > 0.0 && flutter_main_align(l.main_align).is_none_or(|m| !m.contains("Space")) {
+    if l.gap > 0.0 && !matches!(l.main_align, MainAlign::SpaceBetween | MainAlign::SpaceAround | MainAlign::SpaceEvenly) {
         out.push_str(&format!("{inner}spacing: {},\n", num(l.gap)));
     }
     out.push_str(&format!("{inner}children: [\n"));