feat(test): add multi-engine integration matrix

tests/Models/Post.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Tests\Models;
+
+use Libs\Model;
+
+/**
+ * Post - DuckBrain test fixture
+ *
+ * Second table of the integration matrix, joined to Users through user_id.
+ * Column names avoid SQL reserved words on every engine.
+ */
+final class Post extends Model
+{
+    public ?int $id;
+    public ?int $userId;
+    public string $title;
+    public ?string $content;
+    public ?float $rating;
+}

tests/Models/User.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Tests\Models;
+
+use DateTime;
+use Libs\Model;
+
+/**
+ * User - DuckBrain test fixture
+ *
+ * Public model used by the integration matrix. $id is declared WITHOUT a
+ * default so it stays uninitialized until the database assigns it:
+ * Model::getVars() skips uninitialized typed properties, which lets
+ * AUTOINCREMENT / AUTO_INCREMENT / SERIAL generate the primary key and
+ * Model::add() assign it back to the instance.
+ */
+final class User extends Model
+{
+    public ?int $id;
+    public string $username;
+    public ?string $email;
+    public ?string $bio;
+    public ?int $age;
+    public bool $isActive = false;
+    public ?DateTime $lastLogin;
+}