feat: add reactive controllers for threshold and pattern matching

[jimsynz]

Summary

• Add off-the-shelf controllers that monitor PubSub messages and trigger actions when conditions are met
• BB.Controller.PatternMatch - triggers when messages match a predicate function
• BB.Controller.Threshold - convenience wrapper for numeric field threshold monitoring
• BB.Controller.Action - action system with command/1,2 and handle_event/1 builders
• Add module groups to ex_doc for better documentation organisation

Example Usage

defmodule MyRobot do
  use BB

  # Disarm if servo current exceeds limit
  controller :over_current, {BB.Controller.Threshold,
    topic: [:sensor, :servo_status],
    field: :current,
    max: 1.21,
    action: command(:disarm)
  }

  # React to proximity sensor
  controller :collision, {BB.Controller.PatternMatch,
    topic: [:sensor, :proximity],
    match: fn msg -> msg.payload.distance < 0.05 end,
    action: command(:disarm)
  }
end

Test plan

• Unit tests for BB.Controller.Action (7 tests)
• Unit tests for BB.Controller.PatternMatch (7 tests)
• Unit tests for BB.Controller.Threshold (10 tests)
• All existing tests pass
• mix check --no-retry passes