The main user-facing module: a GenServer that manages a single PWM output and caches its state.

By default it talks to the application-supervised Pwmx.Backend. Pass a :backend option to bind it to a specific backend instance instead, this is how you drive an isolated virtual board in tests:

{:ok, backend} = Pwmx.Backend.start_link(name: :my_board, mode: :virtual)
{:ok, pid} = Pwmx.Output.start_link({"virtualchip0", 1}, backend: backend)