feat(scala): m01–m06 port with shared fixtures

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scala/solutions/Random.scala

@@ -0,0 +1,36 @@
+package snake
+
+/**
+ * m04 random: a tiny "state monad" for randomness. A Random[A] is a function seed => (A, nextSeed),
+ * so every draw is a pure function and the seed is passed along explicitly instead of hidden.
+ * Seeds and values are unsigned 32-bit numbers kept in a Long.
+ */
+final case class Random[A](run: Long => (A, Long)):
+
+  def map[B](f: A => B): Random[B] =
+    Random { seed =>
+      val (a, next) = run(seed)
+      (f(a), next)
+    }
+
+  def flatMap[B](f: A => Random[B]): Random[B] =
+    Random { seed =>
+      val (a, next) = run(seed)
+      f(a).run(next)
+    }
+
+object Random:
+
+  def pure[A](a: A): Random[A] = Random(seed => (a, seed))
+
+  // The mulberry32 arithmetic is done in Int: `*` wraps like 32-bit multiplication and `>>>` is unsigned.
+  // `& 0xFFFFFFFFL` turns the result back into an unsigned value held in a Long.
+  lazy val nextUint32: Random[Long] = Random { seed =>
+    val s = seed.toInt + 0x6d2b79f5
+    val t1 = (s ^ (s >>> 15)) * (s | 1)
+    val t2 = t1 ^ (t1 + (t1 ^ (t1 >>> 7)) * (t1 | 61))
+    val value = t2 ^ (t2 >>> 14)
+    (value.toLong & 0xffffffffL, s.toLong & 0xffffffffL)
+  }
+
+  def intBelow(n: Int): Random[Int] = nextUint32.map(v => (v % n).toInt)