Session Store is shared by three groups of code, so a change to it reaches all of them

Session Store holds the SessionStore protocol, the in-memory store and the option validation. Seven parts of the SDK depend on it: both clients and the package exports (public API and internal client), the session mutations, resume, listing and reading modules, and the transcript mirror batcher that appends entries. A change to the protocol or its validation lands in all three groups at once.

Impact: Changing a store method or the validation rules forces coordinated edits across the public API, session management and the one-shot query path.

Recommendation (medium effort): Treat the SessionStore protocol as a stable contract: add methods only with defaults, and keep validate_session_store_options and _store_implements as the single place that checks what a store supports.