fix: add missing assertNotNull import and byte cast in ExecutableDetectorTest

src/test/java/org/dostalgia/ExecutableDetectorTest.java

@@ -9,6 +9,7 @@ import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -141,7 +142,7 @@ class ExecutableDetectorTest {
         // Windows PE executable: MZ header + PE signature at offset 0x80
         byte[] windowsExe = new byte[0x100];
         windowsExe[0] = 0x4D; windowsExe[1] = 0x5A; // MZ
-        windowsExe[0x3C] = 0x80; // PE offset at 0x80
+        windowsExe[0x3C] = (byte) 0x80; // PE offset at 0x80
         windowsExe[0x80] = 0x50; windowsExe[0x81] = 0x45; // "PE" signature
         Files.write(dir.resolve("WIN.EXE"), windowsExe);