• fix: spine ~/halt and move_absolute cancel did not stop the carriage.
  halt_motion() posted to /spine/api/motion:halt, which the device does
  not implement (404). It now uses motion:quick-stop, re-arms to
  SwitchedOn after the DS402 stop, overlaps halt and feedback with the
  blocking motion-mm:start on one HTTP client, and does not report a goal
  canceled when halt fails.
• fix: spine http_timeout (default 3 s) is only the connect timeout and the
  budget for short REST. motion-mm:start uses (http_timeout, 600 s) so
  a long move is not aborted by the short read; the motion lock stays held
  until that POST returns.
• refactor: spine REST wire strings live in SpineStatus (DS402 states) and
  SpineMotion (Finished).
• fix: move_absolute no longer surfaces HTTP 424 when the spine is not
  SwitchedOn. It checks state first and reports e.g.
  Cannot start motion: spine is SwitchedOff (expected SwitchedOn).