Supervises the tier-4 plugins' lifecycle.

On init it runs each plugin's lifecycle.on_start MFA in order (an error return bubbles up and fails boot loud, per the spec), then supervises the plugins' declared lifecycle.supervised child specs alongside the Mob.Plugins.Lifecycle event dispatcher. Started from Mob.App.start/0 via Mob.Plugins.start/0, after the plugins' OTP applications (so an on_start can rely on its own supervision tree and deps) and just before the host's own on_start/0 (so a host on_start that blocks on a run loop can't starve plugin startup — the framework services a plugin's on_start needs are already up), and only when a plugin declares a :lifecycle.