Process-scoped overlay store for tests, backed by NimbleOwnership.

Flag state is owned per-process and resolved through the [self() | $callers] chain (the same mechanism Mox and Ecto.Adapters.SQL.Sandbox use), so spawned Tasks and LiveView processes inherit their test's overrides. A flag with no override resolves to an empty (disabled) flag — overrides overlay on that static default; no global mutable store is consulted, so async: true tests never bleed into each other and flag writes never touch a database or fire notifications.

Configure it as the active store in the test environment:

# config/test.exs
config :bandera, store: Bandera.Store.ProcessScoped

and start the ownership server once in test/test_helper.exs via Bandera.Test.start/0. Per-test cleanup is automatic — NimbleOwnership monitors the owning process and drops its state when the test process exits.