Test helpers for toggling Bandera flags with per-test, async-safe isolation.

Backed by Bandera.Store.ProcessScoped (NimbleOwnership). Setup:

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

# test/test_helper.exs
Bandera.Test.start()

# a test module
defmodule MyTest do
  use ExUnit.Case, async: true
  use Bandera.Test

  @tag feature_flags: [my_flag: true]
  test "feature on" do
    assert Bandera.enabled?(:my_flag)
  end

  test "toggle in the body" do
    enable_flag(:other)
    assert Bandera.enabled?(:other)
  end
end

Overrides are scoped to the test process (and its $callers), so tests run async: true without bleeding into each other, and enable_flag/disable_flag never touch a database or fire notifications. Cleanup is automatic when the test process exits (NimbleOwnership monitors owners); reset/0 clears overrides explicitly within a test if needed.

The use Bandera.Test macro imports enable_flag/1,2 and disable_flag/1,2 for unqualified use. The remaining helpers — put_flag/2,3, clear/1, and reset/0 — are called fully qualified, e.g. Bandera.Test.reset().

Consumers must add {:nimble_ownership, "~> 1.0", only: :test} to their deps.