Integrate logos-chat with the LEZ-backed mix network:
- Update vendor/nwaku to logos-delivery fork (feat/mix-rln-gifter-sim)
with mix protocol, OnchainLEZGroupManager, RLN gifter client, and
mix-rln-spam-protection-plugin
- Add vendor/logos-lez-rln submodule for reproducible LEZ module builds
- Add RLN FFI surface to liblogoschat: chat_set_rln_fetcher,
chat_set_rln_config, chat_push_roots, chat_push_proof, etc.
- Wire gifter client for RLN membership registration via mix network
- Switch to filter-based message reception (relay: false, filter: true)
- Send via lightpushPublish(mixify=true) for Sphinx onion routing
- Add mix-librln separate build + duplicate symbol resolution
- Exclude vendor/logos-lez-rln from nimble link creation
- Update nix build for mix-librln and cross-platform support