Push genesis block to new peers after handshake to help late joiners

[AlanRuno]

Summary

Fixes genesis block propagation issue where late-joining nodes never receive the genesis block. Previously, the genesis block was only broadcast once when created, but if only 1-2 peers were connected at that time, other nodes would never receive it.

This change adds a mechanism to broadcast the genesis block whenever a new peer completes handshake, helping late joiners receive the genesis block. Includes rate limiting (30s per peer address) to reduce redundant broadcasts.

Key changes:

• Added push_genesis_to_peer() function that broadcasts genesis block after handshake completion
• Added rate limiting tracking arrays to prevent flooding
• Called at end of handle_handshake_message() after successful handshake

Review & Testing Checklist for Human

• IMPORTANT: Verify the broadcast-to-all behavior is acceptable. The function broadcasts to ALL peers (via mxd_broadcast_block), not just the newly connected peer. This means every new peer connection triggers a network-wide genesis broadcast (subject to rate limiting).
• Verify rate limiting logic is sufficient - currently tracks by peer address, but broadcasts go to everyone. Multiple different peers connecting in quick succession will each trigger a full broadcast.
• Test with 10-node network: reset all nodes, start them, verify all 10 reach Height: 1 (not just 2-3 nodes)

Recommended test plan:

1. Run ./letsgo testnet -reset on all 10 test nodes
2. Monitor logs for "Pushing genesis block to network after new peer" messages
3. Verify all 10 nodes show Height: 1 within 2-3 minutes

Notes

Link to Devin run: https://app.devin.ai/sessions/f0a6357d898e440ea3745d77dfb91d8d
Requested by: Runo (runonetworks@gmail.com) / @AlanRuno

This is part of ongoing work to fix genesis block propagation. Previous fixes (PR #226) addressed DHT peer port issues but genesis still wasn't reaching all nodes because the initial broadcast happened too early.

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