fix(python): fan out only on JSON-array output, not any IEnumerable

The old code iterated every IEnumerable, so `output = "hello"` emitted one
item per character (a string is IEnumerable-of-char) and `output = {"a":1}`
emitted the keys as items. Now the script's output is serialized first and
the ValueKind decides: JSON array → per-element items; anything else →
single item. `output = None` remains an empty emission.

New tests cover string, dict, number, and None shapes.

src/VolcanicCore.Nodes/PythonTransformNode.cs

@@ -59,12 +59,19 @@ public sealed class PythonTransformNode : IActionNode
                 string.IsNullOrEmpty(formatted) ? ex.Message : formatted);
         }
 
-        if (!scope.TryGetVariable("output", out object? outputObj))
+        if (!scope.TryGetVariable("output", out object? outputObj) || outputObj is null)
             return Task.FromResult<IReadOnlyList<IReadOnlyList<Item>>>(new[] { Array.Empty<Item>() });
 
+        // Fan out only when `output` serializes to a JSON array. A naive
+        // `IEnumerable` check would treat a bare string as an iterable of chars
+        // (`output = "hello"` → 5 items) and a bare dict as an iterable of keys.
+        // Serializing first and dispatching on ValueKind gives one predictable rule.
+        var element = JsonToPython.ToJson(outputObj);
         var outItems = new List<Item>();
-        if (outputObj is System.Collections.IEnumerable ie)
-            foreach (var v in ie) outItems.Add(new Item(JsonToPython.ToJson(v)));
+        if (element.ValueKind == JsonValueKind.Array)
+            foreach (var el in element.EnumerateArray()) outItems.Add(new Item(el.Clone()));
+        else
+            outItems.Add(new Item(element));
 
         return Task.FromResult<IReadOnlyList<IReadOnlyList<Item>>>(new[] { outItems });
     }