fix: Scala 2.12 compatibility issues

- DPMeans.scala: Replace maxOption with if/else max, ArrayBuffer.from with varargs
- Constraints.scala: Replace view.mapValues with map pattern match
- SECrossJoinAssignment.scala: Remove ArraySeq import (2.13 only), use simple Seq

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: derrickburns

src/main/scala/com/massivedatascience/clusterer/ml/DPMeans.scala

@@ -336,13 +336,10 @@ class DPMeans(override val uid: String)
         )
 
         // Check convergence (max center movement)
-        val maxMovement = centers
-          .zip(newCentersArray)
-          .map { case (old, neu) =>
-            kernel.divergence(Vectors.dense(old), Vectors.dense(neu))
-          }
-          .maxOption
-          .getOrElse(0.0)
+        val movements   = centers.zip(newCentersArray).map { case (old, neu) =>
+          kernel.divergence(Vectors.dense(old), Vectors.dense(neu))
+        }
+        val maxMovement = if (movements.isEmpty) 0.0 else movements.max
 
         logInfo(f"Iteration $iter: max center movement = $maxMovement%.6f")
 
@@ -351,7 +348,7 @@ class DPMeans(override val uid: String)
           logInfo(s"Converged after $iter iterations (movement $maxMovement < tol $tolVal)")
         }
 
-        centers = ArrayBuffer.from(newCentersArray)
+        centers = ArrayBuffer(newCentersArray: _*)
       }
 
       assigned.unpersist()

src/main/scala/com/massivedatascience/clusterer/ml/df/Constraints.scala

@@ -75,7 +75,7 @@ class ConstraintSet(constraints: Seq[Constraint]) extends Serializable {
       builder.getOrElseUpdate(ml.i, mutable.Set.empty) += ml.j
       builder.getOrElseUpdate(ml.j, mutable.Set.empty) += ml.i
     }
-    builder.view.mapValues(_.toSet).toMap
+    builder.map { case (k, v) => k -> v.toSet }.toMap
   }
 
   private val cannotLinkIndex: Map[Long, Set[Long]] = {
@@ -84,7 +84,7 @@ class ConstraintSet(constraints: Seq[Constraint]) extends Serializable {
       builder.getOrElseUpdate(cl.i, mutable.Set.empty) += cl.j
       builder.getOrElseUpdate(cl.j, mutable.Set.empty) += cl.i
     }
-    builder.view.mapValues(_.toSet).toMap
+    builder.map { case (k, v) => k -> v.toSet }.toMap
   }
 
   // Constraint weights for penalty calculation

src/main/scala/com/massivedatascience/clusterer/ml/df/strategies/impl/SECrossJoinAssignment.scala

@@ -61,8 +61,8 @@ private[df] class SECrossJoinAssignment extends AssignmentStrategy with Logging
     // Find minimum distance cluster for each point
     // Use window function for efficiency
     import org.apache.spark.sql.expressions.Window
-    import scala.collection.immutable.ArraySeq
-    val windowSpec = Window.partitionBy(ArraySeq.unsafeWrapArray(df.columns.map(col)): _*)
+    val partitionCols = df.columns.map(col)
+    val windowSpec    = Window.partitionBy(partitionCols: _*)
 
     val withRank = withDistances
       .withColumn("rank", row_number().over(windowSpec.orderBy("distance")))