Mint initial tokens for genesis validators via coinbase transactions

[AlanRuno]

Summary

This PR fixes the issue where genesis validators had 0 MXD stake despite the initial_stake config being set to 100.0 MXD. The root cause was that the genesis block creation code was setting total_supply = 0 without actually minting any tokens.

Changes:

• Added serialize_coinbase_for_block() helper function to serialize coinbase transactions for block storage
• Modified mxd_try_coordinate_genesis_block() to create coinbase transactions for each of the 3 genesis validators using the initial_stake config value
• Set genesis_block.total_supply to reflect the total minted tokens (3 × initial_stake)

The coinbase transactions are added to the genesis block before the transaction set is frozen, ensuring they are included in the block's merkle root calculation.

Review & Testing Checklist for Human

• Verify serialization format: Compare serialize_coinbase_for_block() with the existing serialize_transaction_for_block() in mxd_block_proposer.c to ensure the format matches what the deserialization code expects
• Verify UTXO recognition: Confirm that the UTXO system will properly index these coinbase outputs so validators can actually spend their initial stake
• Test on live network: Deploy to test nodes and verify:
  • Genesis block shows total_supply = 30000000000 (300 MXD in base units for 3 validators × 100 MXD each)
  • Each genesis validator shows 100.0 MXD stake in metrics
  • Validators can use their stake for consensus participation
• Verify addresses[] array: Confirm that at line 2264, the addresses[] array contains the correct sorted genesis validator addresses

Recommended test plan:

1. Destroy existing test VMs
2. Pull latest main on MXDTestSuite
3. Run a fresh 10-node test: ./build/mxd_test_suite test --project mxd-network --nodes 10 --startup-mode letsgo-testnet --network-type testnet
4. After genesis block creation, SSH into nodes and verify stake amounts in metrics output

Notes

• The first genesis block creation (for membership digest calculation) is intentionally not modified - it's only used for signing, not for the actual blockchain
• Build passes locally with all tests compiling successfully

Link to Devin run: https://app.devin.ai/sessions/4f2bfeca9bee46ca90ac744cb8b94bf8
Requested by: @AlanRuno

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring