Fix Scala 3.8.4 compilation compatibility while preserving 2.13 and 3.3.x cross-build

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Motivation

The codebase needs to compile with both Scala 3.3.x (current production) and Scala 3.8.4 (forward compatibility for the upcoming 3.9 LTS). Several patterns in the codebase break under Scala 3.8 due to changes in how context bounds are desugared (they become using clauses), stricter deprecation warnings, and syntax changes.

Modification

170 files changed across the codebase:

• ClassTag context bound fixes: In Scala 3.8, [T: ClassTag] desugars to a using clause, making explicit ClassTag passing positional-incompatible. Fixed by using implicit val ct: ClassTag[T] = ClassTag(clazz) in scope and relying on implicit resolution instead of explicit passing. Key files: PersistencePlugin.scala, EventSourcedBehaviorTestKit.scala, ActorContextSpec.scala, BehaviorTestKitSpec.scala, Behavior.scala, EventStream.scala, Topic.scala, ShardingProducerController.scala, ShardedDaemonProcessImpl.scala, StreamTestKit.scala.

• PersistencePlugin implicit parameter merge: Merged [T: ClassTag] context bound and (implicit ev: PluginProvider[...]) into a single implicit parameter list (implicit ct: ClassTag[T], ev: PluginProvider[T, ScalaDsl, JavaDsl]) to avoid context bound + separate implicit parameter mismatch in Scala 3.8.

• .apply() trick: Used decoration[T].apply(behavior) instead of decoration[T](behavior) to separate implicit resolution from function application (Scala 2.13 interprets (behavior) as the implicit ClassTag argument).

• private[this] → private: Scala 3.8 deprecates private[this] in favor of private.

• = _ initializers → explicit defaults: Replaced = _ with = null, = None, = 0, etc. as Scala 3.8 deprecates uninitialized vars.

• SortedSet.empty(ordering) fix: Bound ordering as implicit val ord before calling .empty[Member] to avoid implicit resolution issues.

• @unchecked annotations: Added for type-test patterns that can't be checked at runtime under Scala 3.

• @nowarn filters: Used message-based @nowarn("msg=never used") instead of category-based filters for cross-version compatibility.

• PekkoDisciplinePlugin.scala: Added -Wconf suppression rules for Scala 3.8 deprecation/syntax warnings in both main and test scopes.

• No using keyword in shared sources: All changes use cross-compatible patterns (implicit val, .apply() trick). The using keyword only appears in scala-3/ directories.

Result

All modules compile with both Scala 2.13.18 and Scala 3.8.4. Production Scala 3 version remains at 3.3.8 — this change only ensures forward compatibility. No behavioral changes to the runtime.

Tests

• sbt "++3.8.4" "Test / compile" — success across all modules
• sbt "Test / compile" (Scala 2.13.18) — success across all modules

References

Supersedes #3060 (rebased onto main to resolve Hub.scala conflict with #3063).

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Although we don't know when we'll be able to switch to 3.9.x, it is always a good idea to eliminate compilation warnings and ensure compatibility in advance.