fix: add validation to HopsStamp and DelayStamp

HopsStamp:
- Validate hops amount must be >= 1 (positive integer)
- Negative or zero hops don't make logical sense

DelayStamp:
- Validate delay must be >= 0 (non-negative integer)
- Negative delays don't make logical sense

Both now throw InvalidArgumentException for invalid values, making
configuration errors fail fast with clear messages.

tests/Prime/Stamp/DelayStampTest.php

@@ -26,4 +26,19 @@ final class DelayStampTest extends TestCase
 
         $this->assertSame($this->testDelay, $delay);
     }
+
+    public function testZeroDelayIsValid(): void
+    {
+        $stamp = new DelayStamp(0);
+
+        $this->assertSame(0, $stamp->getDelay());
+    }
+
+    public function testNegativeDelayThrowsException(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Delay must be a non-negative integer (>= 0).');
+
+        new DelayStamp(-100);
+    }
 }

tests/Prime/Stamp/HopsStampTest.php

@@ -20,4 +20,27 @@ final class HopsStampTest extends TestCase
 
         $this->assertSame($initialAmount, $hopsStamp->getAmount());
     }
+
+    public function testZeroHopsThrowsException(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Hops amount must be a positive integer (>= 1).');
+
+        new HopsStamp(0);
+    }
+
+    public function testNegativeHopsThrowsException(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Hops amount must be a positive integer (>= 1).');
+
+        new HopsStamp(-5);
+    }
+
+    public function testMinimumValidHops(): void
+    {
+        $stamp = new HopsStamp(1);
+
+        $this->assertSame(1, $stamp->getAmount());
+    }
 }