Mix has been extracted from nim-libp2p into its own package at
logos-co/nim-libp2p-mix. Update all imports of libp2p/protocols/mix*
to use libp2p_mix*, add the new package as a vendored submodule and
nimble dep, and bump the mix-rln-spam-protection-plugin submodule to
its corresponding migration branch.
Verified end-to-end with simulations/mixnet (5-node mixnet + chat2mix):
- All 5 wakunode2 instances mount /mix/1.0.0 successfully
- RLN spam-protection loads and registers
- chat2mix publishes via lightpush with mixify=true
- Cover traffic runs at the configured 20s interval
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