Shard-local write handlers.
These handlers remain for custom/direct instances and for the staged migration
away from using Shard as a pass-through default quorum write proxy. For the
default application instance, write ingress should go through
Ferricstore.Store.Router or Ferricstore.Raft.Backend and be serialized by
the selected Raft state machine.
Do not add new default-instance write paths here. If a command is durable and user-visible, route it to the Raft backend so read-modify-write behavior is decided in apply order.