net-rs: document Phase 4 Leios protocol implementation plan
Sub-phases 4a-4f: LeiosNotify/LeiosFetch protocols, Praos header/body extensions, per-peer task integration, coordinator/ChainStore extensions, and priority scheduling. Working assumptions: protocol IDs 18/19, opaque types, two protocols initially, freshest-first deferred. Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>