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 general async tasks)
├── DynamicSupervisor (for command GenServers, temporary restart)
├── Runtime (robot state, state machine, command execution)
├── BB.BridgeSupervisor (:one_for_one)
│   └── MavlinkBridge, PhoenixBridge...
└── BB.TopologySupervisor (:one_for_one)
    ├── BB.SensorSupervisor (:one_for_one)
    │   └── RobotSensor1, RobotSensor2...
    ├── BB.ControllerSupervisor (:one_for_one)
    │   └── Controller1, Controller2...
    └── 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. The topology supervisor groups all hardware-facing subtrees so they share a restart budget; when that budget is exhausted the safety controller force-disarms the robot.