feat: add simulation mode for running robots without hardware

[jimsynz]

Summary

• Add runtime simulation mode via start_link(simulation: :kinematic) allowing robots to run without physical hardware
• Actuators are replaced with BB.Sim.Actuator which publishes BeginMotion messages with timing based on joint velocity limits
• Controllers and bridges support per-component simulation DSL option (:omit, :mock, :start)
• Safety system works normally in simulation - must arm before commands
• OpenLoopPositionEstimator works unchanged for position feedback

Usage

# Start in simulation mode
MyRobot.start_link(simulation: :kinematic)

# Check simulation mode at runtime
BB.Robot.Runtime.simulation_mode(MyRobot)  # => :kinematic

# Per-controller/bridge simulation behaviour in DSL
controllers do
  controller :pca9685, {BB.Servo.PCA9685.Controller, bus: "i2c-1"},
    simulation: :omit  # :omit (default), :mock, or :start
end

parameters do
  bridge :mavlink, {BBMavLink.ParameterBridge, conn: "/dev/ttyACM0"},
    simulation: :omit
end

Test plan

• Robot starts in simulation mode with simulation: :kinematic
• Robot starts in hardware mode by default
• Simulated actuator publishes BeginMotion on position command
• Simulated actuator clamps position to joint limits
• Synchronous position command returns acknowledgement
• Controller with simulation: :omit is not started in simulation mode
• Controller with simulation: :omit is started in hardware mode
• Controller with simulation: :mock starts mock controller
• Controller with simulation: :start starts real controller
• Bridge with simulation: :omit is not started in simulation mode
• Bridge with simulation: :omit is started in hardware mode
• Bridge with simulation: :mock starts mock bridge
• Bridge with simulation: :start starts real bridge
• All 555 tests pass
• Dialyzer, credo, formatter all pass