Constant SHARD_LEASES 

pub const SHARD_LEASES: u32 = 64;

Shards are leased one at a time, each with its own epoch. HELLO carries, after the offer, the most shards the worker takes at once (0: all). The lease parameter is the shards granted now: count u32, then shard u32, epoch u64 each; later grants arrive as GRANT frames, and every shard request carries its shard’s epoch. A worker without it waits until every shard is free and gets them all under one epoch.