p2p: harden networking for WAN (NAT, retries, multi-hop gossip guarantees)

[TheNewAutonomy]

Goal

Improve WAN robustness for public testnets/mainnet.

Scope

• NAT traversal strategy (or documented requirement)
• Connection management and backoff
• Ensure transaction propagation does not rely on a single hop from RPC node to leader
• DoS/spam resistance at the networking layer

Acceptance criteria

• Under churn and partial partitions, network continues producing LSUs
• Tx propagation success rate measured and meets target on WAN testnet

[TheNewAutonomy]

Improved WAN tx propagation in dcf5e5e.

• When a node first accepts a newly-seen tx into its mempool, it proactively rebroadcasts the envelope to help multi-hop propagation on WAN (in addition to existing periodic rebroadcast of persisted mempool).

Commit: dcf5e5e