Fixes#3635
It is for v2.0.0 and breaks the ABI.
CircularBuffer::head_ and tail_ are absolute sample counters that grow
forever. As int32_t they overflow after 2^31 samples (~37 h at 16 kHz);
tail_ % capacity then goes negative and std::copy() writes to an invalid
iterator, crashing long-running VAD streams.
Widen every absolute stream position to int64_t. Counts and capacities
(Push/Pop/Get's n, Size(), capacity) stay int32_t — they are bounded by
buffer memory, not stream duration.
Fangjun Kuang