docs: drive traffic light from action+sleep instead of DirectionalLight

The previous demo handed every workitem to a per-direction
DirectionalLight GenServer that scheduled `Process.send_after/3` and
called back into `WorkitemTransition.complete_workitem/2`. With PR #89
merged the runner's per-instance LifecycleHooks already give us a hook
point for "after this transition fires" — we no longer need a separate
GenServer holding timer state.

This commit drops `TrafficLight.DirectionalLight` entirely. Each
transition's `action do ... end` body now (a) re-renders the Kino
frames from `event.markings`, (b) `:timer.sleep`s the colour-display
delay, then (c) drives the next transition's workitem to completion via
a small `TrafficLight.drive_next/2` helper. `on_enactment_start`
bootstraps the cycle by driving `turn_green_ew`.

The `TrafficLight.Supervisor` shrinks to a single child — the
`Task.Supervisor` that wraps each action body — making the example
roughly half the size while exercising more of the new DSL surface.

Author: fahchen