docs(adr-007): add Malachite internal architecture analysis

[qj0r9j0vc2]

Summary

• Document that Malachite uses libp2p internally (GossipSub, Kademlia, TCP/QUIC, Noise, Yamux)
• Add channel extensibility section showing how DCL can add custom channels (DclCars, DclAttestations, WorkerBatches, TransactionGossip)
• Include Arc/Circle production validation as reference
• Update consequences section with corrected information (remove incorrect "no QUIC" claim)
• Add references to Arc, libp2p, and Malachite channel source

Context

Investigation of the Malachite codebase revealed that it wraps libp2p internally, making the "Malachite P2P vs libp2p" decision moot - we get libp2p's battle-tested transport while using Malachite's BFT-optimized API.

Test plan

• Review ADR content for accuracy
• Verify diagrams render correctly in markdown