Add multiple inheritance example demonstrating \ arrays

Author: Clemens Vasters

samples/core/12-multiple-inheritance/README.md

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+# Multiple Inheritance Example
+
+This example demonstrates multiple inheritance using the `$extends` keyword with an array of JSON Pointers, as specified in [JSON Structure Core PR 20](https://github.com/json-structure/json-structure/pull/20).
+
+## Key Concepts
+
+### Multiple Inheritance with $extends Array
+
+The `$extends` keyword can accept either:
+- A single JSON Pointer string (e.g., `"#/definitions/Vehicle"`)
+- An array of JSON Pointer strings (e.g., `["#/definitions/Car", "#/definitions/Aircraft"]`)
+
+When an array is provided, the type inherits from all base types. Properties from earlier types in the array take precedence over later ones (first-wins semantics).
+
+### Schema Structure
+
+```
+Vehicle (abstract)
+  ├── make, model, year
+  │
+  ├── Car ($extends Vehicle)
+  │     └── numDoors, fuelType
+  │
+Aircraft
+  └── wingspan, maxAltitude
+
+FlyingCar ($extends [Car, Aircraft])
+  └── flightMode, transitionTime
+  └── Inherits: make, model, year, numDoors, fuelType, wingspan, maxAltitude
+```
+
+### Merged Properties
+
+A `FlyingCar` instance must include:
+- From `Car` (and transitively from `Vehicle`): `make`, `model`, `year`, `numDoors`, `fuelType`
+- From `Aircraft`: `wingspan`, `maxAltitude`
+- Its own: `flightMode`, `transitionTime` (optional)
+
+## Files
+
+- `schema.struct.json` - The JSON Structure schema demonstrating multiple inheritance
+- `example.json` - A valid instance of a FlyingCar

samples/core/12-multiple-inheritance/example.json

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+{
+  "make": "Terrafugia",
+  "model": "Transition",
+  "year": 2024,
+  "numDoors": 2,
+  "fuelType": "hybrid",
+  "wingspan": 8.0,
+  "maxAltitude": 10000,
+  "flightMode": "ground",
+  "transitionTime": 60
+}

samples/core/12-multiple-inheritance/schema.struct.json

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+{
+  "$schema": "https://json-structure.org/meta/core/v0/",
+  "name": "MultipleInheritanceExample",
+  "version": "1.0",
+  "description": "Demonstrates multiple inheritance using $extends with an array of JSON Pointers",
+  "type": "object",
+  "$ref": "#/definitions/FlyingCar",
+  "definitions": {
+    "Vehicle": {
+      "name": "Vehicle",
+      "abstract": true,
+      "type": "object",
+      "properties": {
+        "make": { "type": "string" },
+        "model": { "type": "string" },
+        "year": { "type": "int16" }
+      },
+      "required": ["make", "model", "year"]
+    },
+    "Car": {
+      "name": "Car",
+      "$extends": "#/definitions/Vehicle",
+      "type": "object",
+      "properties": {
+        "numDoors": { "type": "uint8" },
+        "fuelType": { 
+          "type": "string",
+          "enum": ["gasoline", "diesel", "electric", "hybrid"]
+        }
+      },
+      "required": ["numDoors", "fuelType"]
+    },
+    "Aircraft": {
+      "name": "Aircraft",
+      "type": "object",
+      "properties": {
+        "wingspan": { "type": "double", "description": "Wingspan in meters" },
+        "maxAltitude": { "type": "uint32", "description": "Maximum altitude in feet" }
+      },
+      "required": ["wingspan", "maxAltitude"]
+    },
+    "FlyingCar": {
+      "name": "FlyingCar",
+      "description": "A car that can also fly - demonstrates multiple inheritance",
+      "$extends": ["#/definitions/Car", "#/definitions/Aircraft"],
+      "type": "object",
+      "properties": {
+        "flightMode": {
+          "type": "string",
+          "enum": ["ground", "air", "transition"]
+        },
+        "transitionTime": { 
+          "type": "uint16",
+          "description": "Time in seconds to transition between ground and air mode"
+        }
+      },
+      "required": ["flightMode"]
+    }
+  }
+}