Allow $extends to accept an array of JSON Pointers for multiple inheritance

- Updated $extends value to accept either a single JSON Pointer or an array
- Added rule for collision resolution: first-wins (properties from earlier
  types in the array take precedence)
- Added example demonstrating multiple inheritance with BaseHeader and Metadata

This enables composition patterns common in other schema languages like CDDL.

Fixes #18

Author: Clemens Vasters

draft-vasters-json-structure-core.md

@@ -1670,25 +1670,61 @@ is always `true`).
     value is implicitly `true`).
   - Abstract types MAY extend other abstract types via `$extends`.
 
-### The `$extends` Keyword {#extends-keyword}
-
-The `$extends` keyword merges all properties from an abstract base type into the
-extending type.
-
-If the type using `$extends` is marked as `abstract` and referenced via
-`$addins`, the composite type _replaces_ the base type in the type model of the
-document.
-
-- **Value**: A JSON Pointer to an abstract type.
-- **Rules**:
-  - The `$extends` keyword MUST only be used in schemas of type `object` and
-    `tuple`.
-  - The value of `$extends` MUST be a valid JSON Pointer that points to an
-    abstract type within the same document.
-  - The extending type MUST merge the abstract type’s properties and constraints
-    and MUST NOT redefine any inherited property.
-
-### The `$offers` Keyword {#offers-keyword}
+### The `$extends` Keyword {#extends-keyword}
+
+The `$extends` keyword merges all properties from one or more abstract base
+types into the extending type.
+
+If the type using `$extends` is marked as `abstract` and referenced via
+`$addins`, the composite type _replaces_ the base type in the type model of the
+document.
+
+- **Value**: A JSON Pointer to an abstract type, or an array of JSON Pointers to
+  abstract types.
+- **Rules**:
+  - The `$extends` keyword MUST only be used in schemas of type `object` and
+    `tuple`.
+  - The value of `$extends` MUST be a valid JSON Pointer or an array of valid
+    JSON Pointers that point to abstract types within the same document.
+  - The extending type MUST merge the abstract type's properties and constraints
+    and MUST NOT redefine any inherited property.
+  - When multiple base types are specified, properties are merged in array
+    order. If multiple base types define a property with the same name, the
+    property from the first base type in the array takes precedence.
+
+**Example**:
+
+~~~ json
+{
+  "definitions": {
+    "BaseHeader": {
+      "type": "object",
+      "abstract": true,
+      "properties": {
+        "version": { "type": "uint32" },
+        "timestamp": { "type": "uint64" }
+      }
+    },
+    "Metadata": {
+      "type": "object",
+      "abstract": true,
+      "properties": {
+        "created": { "type": "uint64" },
+        "modified": { "type": "uint64" }
+      }
+    },
+    "FullRecord": {
+      "type": "object",
+      "$extends": ["#/definitions/BaseHeader", "#/definitions/Metadata"],
+      "properties": {
+        "data": { "type": "binary" }
+      }
+    }
+  }
+}
+~~~
+
+### The `$offers` Keyword {#offers-keyword}### The `$offers` Keyword {#offers-keyword}
 
 The `$offers` keyword is used to advertise add-in types that are available for
 use in a schema document. The `$offers` keyword is a map of add-in names to