Fix ThreadLocalRandom

Current impl was not creating thread local randoms!

lila/src/main/scala/Random.scala

@@ -2,7 +2,12 @@ package scalalib
 
 import scala.collection.mutable.StringBuilder
 
-val ThreadLocalRandom = RandomApi(java.util.concurrent.ThreadLocalRandom.current)
+import scala.collection.mutable.StringBuilder
+
+private final val store =
+  java.lang.ThreadLocal.withInitial(() => RandomApi(java.util.concurrent.ThreadLocalRandom.current))
+
+def ThreadLocalRandom = store.get
 
 val SecureRandom = RandomApi(java.security.SecureRandom())
 

lila/src/test/scala/RandomTest.scala

@@ -0,0 +1,12 @@
+package scalalib
+
+class RandomTest extends munit.FunSuite:
+
+  test("different threads get different randoms"):
+    val r1 = ThreadLocalRandom
+
+    @volatile var r2: RandomApi = null
+    val thread2                 = new Thread(() => r2 = ThreadLocalRandom)
+    thread2.start()
+    thread2.join()
+    assertNotEquals(r1, r2)