refactor: extract findFilesByExtensions to FileUtils

- Added FileUtils.findFilesByExtensions(dir, extensions) — generic
  walk-and-collect utility for matching files by extension
- ExecutableDetector.findAll() now delegates to it, dropping
  13 lines of boilerplate

src/main/java/com/dostalgia/ExecutableDetector.java

@@ -36,19 +36,7 @@ public class ExecutableDetector {
     );
 
     public List<String> findAll(Path dir) throws IOException {
-        List<String> result = new ArrayList<>();
-        Files.walkFileTree(dir, new SimpleFileVisitor<>() {
-            @Override
-            public @Nonnull FileVisitResult visitFile(@Nonnull Path f, @Nonnull BasicFileAttributes a) {
-                String name = f.getFileName().toString().toLowerCase();
-                if (name.endsWith(".exe") || name.endsWith(".com") || name.endsWith(".bat")) {
-                    result.add(dir.relativize(f).toString().replace('\\', '/'));
-                }
-                return FileVisitResult.CONTINUE;
-            }
-        });
-        result.sort(String::compareTo);
-        return result;
+        return FileUtils.findFilesByExtensions(dir, Set.of(".exe", ".com", ".bat"));
     }
 
     public String findMain(Path dir) throws IOException {

src/main/java/com/dostalgia/FileUtils.java

@@ -8,6 +8,9 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.SimpleFileVisitor;
 import java.nio.file.attribute.BasicFileAttributes;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
 
 /** Shared file-system utilities. */
 public final class FileUtils {
@@ -30,4 +33,28 @@ public final class FileUtils {
             }
         });
     }
+
+    /**
+     * Walk a directory tree and collect relative paths of files whose extension matches
+     * one of the given extensions. Extensions should include the dot, e.g. {@code ".exe"}.
+     * Results are sorted alphabetically.
+     */
+    public static List<String> findFilesByExtensions(Path dir, Set<String> extensions) throws IOException {
+        List<String> result = new ArrayList<>();
+        Files.walkFileTree(dir, new SimpleFileVisitor<>() {
+            @Override
+            public FileVisitResult visitFile(Path f, BasicFileAttributes a) {
+                String name = f.getFileName().toString().toLowerCase();
+                for (String ext : extensions) {
+                    if (name.endsWith(ext)) {
+                        result.add(dir.relativize(f).toString().replace('\\', '/'));
+                        break;
+                    }
+                }
+                return FileVisitResult.CONTINUE;
+            }
+        });
+        result.sort(String::compareTo);
+        return result;
+    }
 }