Fix Moonraker mode spurious SET_ACTIVE_SPOOL during CFS slot transitions

During multi-color prints the firmware briefly sends selected=0 for all CFS
slots while switching, which was triggering the SP fallback and calling
SET_ACTIVE_SPOOL with SP's linked spool ID.

- _parse_boxs_info: only call SET_ACTIVE_SPOOL when firmware explicitly
  selects a CFS slot (active_slot non-None); never use SP as a fallback here
- moonraker_job_poll_loop: at job start, if tracking slot is SP, activate
  SP's linked spool — this is the only place SP activation now happens

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>