Root supervisor for a BB robot.

Builds a supervision tree that mirrors the robot topology for fault isolation. A crash in an actuator at the end of a limb only affects that limb's subtree.

Supervision Tree Structure

BB.Supervisor (root, :one_for_one)
├── Registry (named {MyRobot, :registry})
├── PubSub Registry (named {MyRobot, :pubsub})
├── Task.Supervisor (for command execution tasks)
├── Runtime (robot state, state machine, command execution)
├── BB.SensorSupervisor (:one_for_one)
│   └── RobotSensor1, RobotSensor2...
├── BB.ControllerSupervisor (:one_for_one)
│   └── Controller1, Controller2...
├── BB.BridgeSupervisor (:one_for_one)
│   └── MavlinkBridge, PhoenixBridge...
└── BB.LinkSupervisor(:base_link, :one_for_one)
    ├── LinkSensor (link sensors)
    └── BB.JointSupervisor(:shoulder, :one_for_one)
        ├── JointSensor
        ├── JointActuator
        └── BB.LinkSupervisor(:arm, :one_for_one)
            └── ...

Each subsystem supervisor (sensors, controllers, bridges) has its own restart budget, so a flapping process in one won't exhaust the root supervisor's budget and bring down the entire robot.