Constant MAX_WORKER_CHANNEL_CAPACITY 

pub const MAX_WORKER_CHANNEL_CAPACITY: usize = 1_000_000;

Upper bound on worker_channel_capacity. tokio::sync::mpsc::channel does not preallocate slots (it links chunks lazily under a Semaphore), but an oversized value still lets the per-shard buffers grow unboundedly under overload and hides real backpressure signals. 1M is orders of magnitude above realistic capacity — this bound is defence-in-depth against a fat-finger, not a hard OOM prevention.