Add cats integration tests (Eq, Show, coerce)

Tests verify that deriving works with cats typeclasses, confirming
the plugin's output is compatible with cats on both Scala 2.13 and 3.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MateuszKubuszok

tests/src/test/scala/io/estatico/newtype/NewTypeCatsTest.scala

@@ -0,0 +1,38 @@
+package io.estatico.newtype
+
+import io.estatico.newtype.macros._
+import io.estatico.newtype.ops._
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import cats._
+import cats.syntax.all._
+
+class NewTypeCatsTest extends AnyFlatSpec with Matchers {
+
+  import NewTypeCatsTest._
+
+  behavior of "@newtype with cats"
+
+  it should "derive Eq instance" in {
+    implicit val eqInt: Eq[Int] = Eq.fromUniversalEquals
+    val eqFoo: Eq[Foo] = Foo.deriving[Eq]
+    eqFoo.eqv(Foo(1), Foo(1)) shouldBe true
+    eqFoo.eqv(Foo(1), Foo(2)) shouldBe false
+  }
+
+  it should "derive Show instance" in {
+    implicit val showInt: Show[Int] = Show.fromToString
+    val showFoo: Show[Foo] = Foo.deriving[Show]
+    showFoo.show(Foo(42)) shouldBe "42"
+  }
+
+  it should "coerce with .coerce" in {
+    val foo: Foo = 42.coerce[Foo]
+    val i: Int = foo.coerce[Int]
+    i shouldBe 42
+  }
+}
+
+object NewTypeCatsTest {
+  @newtype case class Foo(x: Int)
+}