chore: make code compiles with 3.8.4

[He-Pin]

Motivation

Upgrade Pekko's Scala 3 version from 3.3.8 to 3.8.4 for forward compatibility with the upcoming Scala 3.9 LTS. Several patterns valid in 3.3.x produce errors or warnings in 3.8.4 that must be addressed while preserving cross-compilation with Scala 2.13.

Modification

Scala version:

• Bump scala3Version from 3.3.8 to 3.8.4

PersistencePlugin refactor:

• Merge [T: ClassTag] context bound and (implicit ev: PluginProvider) into a single (implicit ct: ClassTag[T], ev: PluginProvider) parameter list, enabling subclasses to pass both arguments without the using keyword (cross-compatible with Scala 2.13 and 3.3.x)

ClassTag context bound fixes:

• Replace explicit ClassTag passing method(ClassTag(clazz)) with implicit val ct = ClassTag(clazz); method pattern throughout the codebase (context bounds become using clauses in Scala 3.8, making positional passing an error)
• Affects: Behavior, EventStream, Topic, ShardingProducerController, ShardedDaemonProcessImpl, StreamTestKit, EventSourcedBehaviorTestKit, ActorContextSpec

SortedSet/TreeSet ordering fixes:

• Fix SortedSet.empty(ordering) / TreeSet.empty(ordering) across cluster, cluster-tools, cluster-sharding, distributed-data modules by binding the Ordering as an implicit val before calling .empty[Member]

JDK targeting:

• Use -java-output-version for Scala 3.8+ instead of deprecated -release / -Xtarget

Misc fixes:

• Add @unchecked to type-test patterns that cannot be checked at runtime (Reachability.scala, AskPattern.scala)
• Fix _ wildcard type argument to ? in scala-3-only source files
• Fix LogSource resolution in MultiNodeSpec (classOf instead of this.getClass)
• Fix "pure expression does nothing" in FixedBufferSpec structural type calls
• Remove discarded non-Unit value (unused Success) in SnapshotStorage
• Add -Wconf suppression rules for Scala 3.8 deprecation and syntax warnings in PekkoDisciplinePlugin
• Replace = _ with = null / = None / = false (deprecated in Scala 3.4+)
• Replace private[this] / protected[this] with private / protected (deprecated in Scala 3.4+)

Result

All modules compile successfully with both Scala 2.13.18 and Scala 3.8.4. Cross-compilation with Scala 2.13 is preserved. No using keyword is used in shared source files — all changes use cross-compatible patterns.

Tests

• sbt "++3.8.4" "compile" "Test / compile" — success
• sbt "compile" "Test / compile" (Scala 2.13) — success (docs module has pre-existing unused warnings)
• sbt "persistence / mimaReportBinaryIssues" — success
• scalafmt --mode diff-ref=origin/main — success
• sbt headerCreateAll — success

References

None - proactive Scala 3 forward compatibility