calculateSlotCount

        fn calculateSlotCount(size_class_index: usize) SlotIndex {
            const size_class = @as(usize, 1) << @as(Log2USize, @intCast(size_class_index));
            var lower: usize = 1 << minimum_slots_per_bucket_log2;
            var upper: usize = (page_size - bucketSize(lower)) / size_class;
            while (upper > lower) {
                const proposed: usize = lower + (upper - lower) / 2;
                if (proposed == lower) return lower;
                const slots_end = proposed * size_class;
                const header_begin = mem.alignForward(usize, slots_end, @alignOf(BucketHeader));
                const end = header_begin + bucketSize(proposed);
                if (end > page_size) {
                    upper = proposed - 1;
                } else {
                    lower = proposed;
                }
            }
            const slots_end = lower * size_class;
            const header_begin = mem.alignForward(usize, slots_end, @alignOf(BucketHeader));
            const end = header_begin + bucketSize(lower);
            assert(end <= page_size);
            return lower;
        }