fix: auto-index silently never fires on batched increments

[gamandeepsingh]

*count == threshold only matched when the count landed exactly on the threshold, which worked for one-by-one increments but silently dropped entries when a batch flush pushed the count past it in one go (e.g. 0→15 with threshold=10).

Changed to *count < threshold + increment — equivalent to checking previous_count < threshold — so the entry is added to the priority queue on the genuine first crossing, regardless of batch size, without re-triggering after the queue consumes it.

Closes: #2