Add per-key version-locked writes to FfiDynStore

Introduce FfiDynStoreInner with per-key write version locks that
ensure write ordering and skip stale versions in both sync and async
code paths.

Test changes:
- Unify tier store test helpers to use TestSyncStore for all tiers,
  replacing mixed SqliteStore/FilesystemStore/TestStore usage that
  caused test hangs due to TestStore's async write blocking
- Split build_node_with_store into cfg-gated versions for uniffi
  vs non-uniffi feature flags

Author: enigbe