fix: add cross-version parallel collections support

Implement version-specific compat package objects to handle
Scala 2.12/2.13 parallel collections differences:
- Scala 2.12: Empty package object (parallel collections built-in)
- Scala 2.13: Re-export CollectionConverters implicit conversions

This enables .par to work across both Scala versions in KMeansPlusPlus.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: derrickburns

src/main/scala-2.13/com/massivedatascience/clusterer/compat/package.scala

@@ -1,4 +1,13 @@
 package com.massivedatascience.clusterer
 
 // Scala 2.13: Re-export CollectionConverters implicits for .par support
-package object compat extends scala.collection.parallel.CollectionConverters
+package object compat {
+  implicit def asParIterable[A](iterable: Iterable[A]): scala.collection.parallel.CollectionConverters.IterableIsParallelizable[A] =
+    scala.collection.parallel.CollectionConverters.IterableIsParallelizable(iterable)
+
+  implicit def asParSeq[A](seq: scala.collection.Seq[A]): scala.collection.parallel.CollectionConverters.SeqIsParallelizable[A] =
+    scala.collection.parallel.CollectionConverters.SeqIsParallelizable(seq)
+
+  implicit def asParMap[K, V](map: scala.collection.Map[K, V]): scala.collection.parallel.CollectionConverters.MapIsParallelizable[K, V] =
+    scala.collection.parallel.CollectionConverters.MapIsParallelizable(map)
+}