feat: Introduce ElementChildRelation for node participation in the Duit element tree

lib/src/misc/element_child_relation.dart

@@ -0,0 +1,19 @@
+/// Canonical numeric codes for how a node participates in the Duit element tree.
+///
+/// Values are stable wire-level integers (0–4); prefer these names over raw literals.
+sealed class ElementChildRelation {
+  /// No structural child slots (leaf nodes in the tree sense).
+  static const none = 0;
+
+  /// Exactly one structural child.
+  static const single = 1;
+
+  /// Multiple structural children.
+  static const multi = 2;
+
+  /// Component slot (merged template / component content).
+  static const component = 3;
+
+  /// Fragment slot (registered fragment subtree).
+  static const fragment = 4;
+}

lib/src/misc/index.dart

@@ -1,3 +1,4 @@
+export "element_child_relation.dart";
 export "script_runner.dart";
 export "logger/index.dart";
 export "parser.dart";

lib/src/ui/descriptor.dart

@@ -35,11 +35,7 @@ final class WidgetDescriptor {
 
   /// Defines the child relationship pattern for this element type.
   ///
-  /// - `0`: No child elements (leaf widgets like Text, Image)
-  /// - `1`: Single child element (wrapper widgets like Container, Padding)
-  /// - `2`: Multiple child elements (layout widgets like Row, Column, Stack)
-  /// - `3`: Component content
-  /// - `4`: Fragment content
+  /// Prefer [ElementChildRelation] constants instead of raw integers.
   final int childRelation;
 
   /// Whether this element type is controlled by default.