Add pluggable Serializer interface for better portability

[sfloess]

Proposal

Currently uses Java built-in ObjectOutputStream/ObjectInputStream. This has drawbacks:

• Not portable across JVM versions
• Security concerns with deserialization
• Large binary size

Suggestion: Introduce a Serializer<T> interface with default Java implementation, and support for JSON (Jackson/Gson), Protobuf, etc. Make it configurable in Builder.

Benefits: Better archival, cross-language potential, smaller footprints.