Add Virtual Threads Tests, Fix Executor Shutdown Leak, and Optimize Predecessor Lookup

src/main/java/dev/shaaf/jgraphlet/CacheKey.java

@@ -1,10 +1,27 @@
 package dev.shaaf.jgraphlet;
 
+import java.util.*;
+
 /**
  * Cache key used to uniquely identify the output of a task for a given input.
  *
  * @param taskName the name of the task
  * @param input    the input provided to that task
  */
 public record CacheKey(String taskName, Object input) {
-}
+  public CacheKey {
+    Objects.requireNonNull(taskName, "taskName");
+    input = normalize(input); // snapshot for stable equals/hashCode
+  }
+  private static Object normalize(Object o) {
+    if (o == null) return null;
+    if (o instanceof Map<?, ?> m) {
+      // preserve iteration order to keep equals/hash stable
+      var copy = new LinkedHashMap<>(m);
+      return Map.copyOf(copy);
+    }
+    if (o instanceof List<?> l) return List.copyOf(l);
+    if (o instanceof Set<?> s)  return Set.copyOf(s);
+    return o; // assume immutable or stable
+  }
+}