- Introduce workerState holding consecutiveEmpty and consecutiveErrors
- Plumb workerState through pollOnce, fetchTask and calculateInterval
- Drop the shared atomic.Int64 counters from Poller
With Capacity > 1, the previous shared counters inflated whenever multiple
workers each saw a single empty response, triggering an unnecessarily long
backoff. Per-worker state keeps each goroutine's backoff independent.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
