feat(core): Implement DDD, CQRS, and Event Sourcing architecture

- Add project README and Composer configuration with PSR-4 autoloading
- Implement domain layer with AggregateRoot base class for event sourcing
- Create Carrier aggregate with CarrierRegistered domain event
- Add application layer with RegisterCarrierCommand and RegisterCarrierHandler
- Implement infrastructure layer with EventStore interface and CassandraEventStore
- Add CommandBus and Router for request handling and routing
- Create demo test file to showcase carrier registration workflow
- Establish foundation for event-driven architecture with Cassandra persistence

tests/demo.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace Cassandra {
+    class Timestamp
+    {
+    }
+}
+
+namespace {
+    // Mock Cassandra Session
+    class MockCassandraSession
+    {
+        public $executedStatements = [];
+
+        public function prepare($cql)
+        {
+            return new MockStatement($cql);
+        }
+
+        public function execute($statement, $options)
+        {
+            $this->executedStatements[] = [
+                'statement' => $statement->cql,
+                'arguments' => $options['arguments']
+            ];
+            echo "Executed CQL: " . $statement->cql . "\n";
+            return [];
+        }
+    }
+
+    class MockStatement
+    {
+        public $cql;
+        public function __construct($cql)
+        {
+            $this->cql = $cql;
+        }
+    }
+
+    require_once __DIR__ . '/../src/Infrastructure/EventStore.php';
+    require_once __DIR__ . '/../src/Infrastructure/CassandraEventStore.php';
+    require_once __DIR__ . '/../src/Infrastructure/CommandBus.php';
+    require_once __DIR__ . '/../src/Domain/AggregateRoot.php';
+    require_once __DIR__ . '/../src/Domain/Events/CarrierRegistered.php';
+    require_once __DIR__ . '/../src/Domain/Carrier.php';
+    require_once __DIR__ . '/../src/Application/Commands/RegisterCarrierCommand.php';
+    require_once __DIR__ . '/../src/Application/Handlers/RegisterCarrierHandler.php';
+
+    use DistributingCarriers\Infrastructure\CommandBus;
+    use DistributingCarriers\Infrastructure\CassandraEventStore;
+    use DistributingCarriers\Application\Commands\RegisterCarrierCommand;
+    use DistributingCarriers\Application\Handlers\RegisterCarrierHandler;
+
+    echo "Starting Demo...\n";
+
+    $session = new MockCassandraSession();
+    $eventStore = new CassandraEventStore($session);
+    $commandBus = new CommandBus();
+
+    $commandBus->register(RegisterCarrierCommand::class, new RegisterCarrierHandler($eventStore));
+
+    $command = new RegisterCarrierCommand("Demo Carrier", "demo@example.com");
+    $commandBus->dispatch($command);
+
+    echo "Demo Completed.\n";
+}