feat: add param() references and wrapper GenServer pattern

[jimsynz]

Summary

• Add param() references in actuator/sensor/controller options with automatic resolution and runtime updates
• Add param() references for topology fields (joint limits, link origins, etc.)
• Allow setting parameter values via start_link options
• Implement wrapper GenServer pattern for actuators, sensors, and controllers

Wrapper GenServer Pattern

User modules now define callbacks only. Framework-controlled wrapper GenServers (BB.Actuator.Server, BB.Sensor.Server, BB.Controller.Server) manage the process lifecycle and delegate to user callbacks.

# Before: User modules ARE GenServers
defmodule MyActuator do
  use BB.Actuator
  @impl GenServer
  def init(opts), do: ...
end

# After: User modules define callbacks only
defmodule MyActuator do
  use BB.Actuator,
    options_schema: [pin: [type: :pos_integer, required: true]]

  @impl BB.Actuator
  def init(opts), do: ...

  @impl BB.Actuator
  def handle_options(new_opts, state), do: {:ok, state}
end

param() References

Parameters can now be referenced in actuator/sensor/controller options:

parameters do
  parameter [:motion, :max_effort], type: :float, default: 1.0
end

joint :shoulder do
  actuator :motor, {MyMotor, max_effort: param([:motion, :max_effort])}
end

When the parameter changes, the actuator's handle_options/2 callback is invoked.

Breaking Changes

• @impl GenServer → @impl BB.Actuator (or Controller/Sensor) for all callbacks
• options_schema moved from callback to use macro option
• Safety registration remains explicit via BB.Safety.register/2 in init/1

Test plan

• All 529 tests passing
• Verified param() resolution in child specs
• Verified handle_options callback on param changes
• Updated bb_servo_pigpio, bb_servo_pca9685, bb_servo_robotis (separate PRs)