Fix cfs_stats not updating for boxes 2-4

Filament usage was only attributed to a slot if it existed in st.slots
(manually configured or Spoolman-linked). Slots in boxes 2-4 that
hadn't been set up were silently dropped, so their cfs_stats never
updated.

Fix: store the CFS-reported material type in cfs_slots so it's
available without a local slot object. In the Moonraker poll loop,
fall back to cfs_slots material when the slot isn't in st.slots,
so all active slots across all boxes are tracked correctly.

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