feat: enhance cross-version persistence CI to test all model types

Enhanced persistence-cross job to test all 4 model types in matrix:
- GeneralizedKMeansModel
- KMedoidsModel
- SoftKMeansModel
- StreamingKMeansModel

Each model type now tested across:
- Scala 2.12 ↔ 2.13 (bidirectional)
- Spark 3.4.0 ↔ 3.5.1 (bidirectional)

This validates PersistenceLayoutV1's cross-version compatibility guarantee
for all model types with their unique state (medoids, weights, soft params).

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

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

Author: derrickburns

.github/workflows/ci.yml

@@ -112,10 +112,19 @@ jobs:
         run: sbt ++${{ env.SCALA_213 }} "runMain examples.PersistenceRoundTrip load /tmp/test-model"
 
   # Job 4: Cross-version persistence test (save in 2.12, load in 2.13, vice versa)
+  # Tests all 4 model types: GeneralizedKMeans, KMedoids, SoftKMeans, StreamingKMeans
   persistence-cross:
     runs-on: ubuntu-latest
     needs: build
-    name: Cross-version Persistence
+    strategy:
+      fail-fast: false
+      matrix:
+        algo:
+          - { name: "GeneralizedKMeans", main: "examples.PersistenceRoundTrip" }
+          - { name: "KMedoids", main: "examples.PersistenceRoundTripKMedoids" }
+          - { name: "SoftKMeans", main: "examples.PersistenceRoundTripSoftKMeans" }
+          - { name: "StreamingKMeans", main: "examples.PersistenceRoundTripStreamingKMeans" }
+    name: Cross-version Persistence (${{ matrix.algo.name }})
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
       - uses: actions/setup-java@8df1039502a15bceb9433410b1a100fbe190c53b  # v4.5.0
@@ -129,19 +138,23 @@ jobs:
         with:
           name: jars
           path: target
-      - name: Save model with Scala 2.12
+      - name: Test 2.12 → 2.13 (Scala version compatibility)
         run: |
           mkdir -p /tmp/persistence-test
-          sbt ++${{ env.SCALA_212 }} "runMain examples.PersistenceRoundTrip save /tmp/persistence-test/model-212"
-      - name: Load model with Scala 2.13
+          sbt ++${{ env.SCALA_212 }} "runMain ${{ matrix.algo.main }} save /tmp/persistence-test/model-212"
+          sbt ++${{ env.SCALA_213 }} "runMain ${{ matrix.algo.main }} load /tmp/persistence-test/model-212"
+      - name: Test 2.13 → 2.12 (reverse Scala compatibility)
         run: |
-          sbt ++${{ env.SCALA_213 }} "runMain examples.PersistenceRoundTrip load /tmp/persistence-test/model-212"
-      - name: Save model with Scala 2.13
+          sbt ++${{ env.SCALA_213 }} "runMain ${{ matrix.algo.main }} save /tmp/persistence-test/model-213"
+          sbt ++${{ env.SCALA_212 }} "runMain ${{ matrix.algo.main }} load /tmp/persistence-test/model-213"
+      - name: Test Spark 3.4 → 3.5 (Spark version compatibility)
         run: |
-          sbt ++${{ env.SCALA_213 }} "runMain examples.PersistenceRoundTrip save /tmp/persistence-test/model-213"
-      - name: Load model with Scala 2.12
+          sbt ++${{ env.SCALA_213 }} -Dspark.version=3.4.0 "runMain ${{ matrix.algo.main }} save /tmp/persistence-test/model-spark34"
+          sbt ++${{ env.SCALA_213 }} -Dspark.version=3.5.1 "runMain ${{ matrix.algo.main }} load /tmp/persistence-test/model-spark34"
+      - name: Test Spark 3.5 → 3.4 (reverse Spark compatibility)
         run: |
-          sbt ++${{ env.SCALA_212 }} "runMain examples.PersistenceRoundTrip load /tmp/persistence-test/model-213"
+          sbt ++${{ env.SCALA_213 }} -Dspark.version=3.5.1 "runMain ${{ matrix.algo.main }} save /tmp/persistence-test/model-spark35"
+          sbt ++${{ env.SCALA_213 }} -Dspark.version=3.4.0 "runMain ${{ matrix.algo.main }} load /tmp/persistence-test/model-spark35"
 
   # Job 5: Performance sanity check (ensure no major regression)
   perf-sanity: