In-memory DurableServer.StorageBackend for dev and test.

Drop-in compatible with DurableServer.Backends.ObjectStore, including its JSON round-trip: stored bodies are encoded with JSON.encode!/1, so state comes back with string keys exactly as it would from S3. Etag CAS (try_claim/3, put_object/4 with :etag, update_object/4) is implemented faithfully — DurableServer's locking depends on it.

Storage lives in a named Agent, so it survives DurableServer supervisor restarts (the deploy-simulation scenario). Start it explicitly:

# test
start_supervised!({DurableStash.TestBackend, name: MyBackend})

# dev supervision tree
children = [
  {DurableStash.TestBackend, name: MyBackend},
  {DurableServer.Supervisor,
   name: MySupervisor, prefix: "dev/", backend: {DurableStash.TestBackend, name: MyBackend}}
]

If the named agent is not running when init_backend/1 is called, it is started unlinked as a convenience for one-off scripts.