feat: add BB.Message.Actuator.EndMotion

[jimsynz]

Summary

Adds BB.Message.Actuator.EndMotion - a message that actuators can optionally publish when motion ends. This complements BeginMotion and is useful for actuators with partial feedback (limit switches, stall detection) that can report completion but may not have continuous position sensing.

Fields

• position (float, required) - position when motion ended (radians or metres)
• reason (atom, required) - why motion ended:
  • :completed - motion reached target normally
  • :cancelled - motion was interrupted
  • :limit_reached - hit a limit switch or software limit
  • :fault - error condition (stall, overcurrent, etc.)
• detail (atom, optional) - additional context (e.g. :end_stop, :software, :stall)
• message (string, optional) - human-readable information for operators

Example Usage

alias BB.Message
alias BB.Message.Actuator.EndMotion

# Simple completion
Message.new!(EndMotion, :shoulder, position: 1.57, reason: :completed)

# Limit reached with detail
Message.new!(EndMotion, :shoulder,
  position: 0.0,
  reason: :limit_reached,
  detail: :end_stop
)

# Fault with message
Message.new!(EndMotion, :shoulder,
  position: 0.52,
  reason: :fault,
  detail: :stall,
  message: "Motor stall detected at 30% travel"
)

Test plan

• Tests for message creation with required fields only
• Tests for optional detail field
• Tests for optional message field
• Tests for required field validation
• Tests for reason enum validation
• All 457 tests pass
• mix check --no-retry passes