Add logging.access.sample_rate config (0.0-1.0) for deterministic request sampling. 5xx errors are always logged; 2xx/3xx/4xx follow the configured rate. Uses atomic.Uint64 counter for lock-free, zero-allocation sampling decisions. Includes test updates to verify: - sample_rate=1.0 logs all requests - sample_rate=0.0 logs only 5xx - 5xx are always logged regardless of rate