GET /spoolman/spools/unlinked hid any spool with a non-empty
extra.tag from the AMS-slot assignment picker. extra.tag is only an
RFID/NFC matching key -- OpenSpoolman writes its own NFC tag value
into that same Spoolman field -- so every OpenSpoolman-tagged spool
became un-assignable in Bambuddy even when it occupied no slot.
get_unlinked_spools now determines assignability from the
spoolman_slot_assignments table (the documented source of truth for
slot assignments) and ignores extra.tag entirely. Both link_spool and
the AMS auto-sync upsert a row there for every occupied slot, so the
ledger is complete. get_linked_spools and find_spool_by_tag still use
extra.tag -- they are genuine tag-match maps and are unaffected.
Internal-inventory mode needs no parallel change: it stores tags in
its own DB with no Spoolman extra collision.
Updates test_get_unlinked_spools_success and adds
test_get_unlinked_spools_excludes_slot_assigned.