Parse an effect name into its default Effect.
Accepts Wallr names plus the directional aliases familiar from awww:
simple, left, right, top, bottom, center, any, and random.
Canonical effect names. Directional and convenience aliases (simple,
left, right, top, bottom, center, any, random) still map
through effect_from_name; this list drives CLI help and errors so
users see the six real transitions rather than every alias.