Mem-Client: Hydration Compatibility

Mem-client hydration/default changes must be backward-compatible with existing cache.

Scope:
- Enforce when changing store hydration/default state logic in `domains/frontend/mem-client/src/**`.

Rules:
1) New fields/defaults should handle missing legacy values safely.
2) Hydration changes should avoid regressing existing-user state flows.
3) Include validation evidence for an existing-user path when hydration defaults materially change behavior.

Review behavior:
- Return NOT APPROVED when hydration/default logic changes appear unsafe for existing cached state.
- Do not block additive fields that are clearly safely defaulted.