fc_slot_create, fc_slot_destroy, fc_slot_open, fc_slot_close, and
fc_slot_write_frame were defined in slot.c but never declared in slot.h.
Any translation unit calling them without seeing a proper prototype
would fall back to implicit int return (32 bits), truncating 64-bit
pointers and causing SIGSEGV on dereference.
This affected framecache.c (POST /slots → fc_slot_create, DELETE
→ fc_slot_destroy) and other callers.
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