feat(consensus): evaluate simplex ForwardingPolicy and Relay::Plan

[decofe]

After PR #3554 we set ForwardingPolicy::Disabled and ignore Relay::Plan in the broadcast implementation.

Commonware's simplex engine now supports forwarding certified blocks to peers who missed them — either to all non-voters (Silent) or just the next view's leader (NextLeader). The Plan enum distinguishes initial Propose broadcasts from targeted Forward { round, peers } rebroadcasts.

We should:

1. Evaluate whether enabling forwarding improves liveness under real-world conditions (missed proposals, slow validators)
2. If so, implement plan-aware relay logic that handles Forward by sending only to the specified peers
3. Pick the right policy (Silent vs NextLeader) based on our validator set size and network profile