Phase 8: Enforce ACL on connection and gossip layer

[realnatesullivan]

Sub-task of #10.\n\nIntegrate ACL enforcement at the connection acceptance and gossip message processing layers.\n\nAcceptance criteria:\n- [ ] Connections from denied peers rejected at handshake\n- [ ] Gossip messages from denied peers dropped\n- [ ] Allowlist-only mode supported\n- [ ] Unit and integration tests