Add HTTP API transaction endpoint and give all genesis validators 100 MXD

[AlanRuno]

Summary

This PR makes two changes to support 10-node test network deployment:

1. Genesis block modification: Changed genesis block creation to give ALL participating validators 100 MXD each, instead of only the first 3 validators. This enables proper testing with larger validator sets.

2. HTTP API transaction endpoints: Added new endpoints for transaction submission and balance checking:

   • POST /transaction - Submit a signed transaction (requires from, to, amount, private_key, public_key, algo_id)
   • GET /balance/{address} - Check balance for an address (40 hex chars)
   • Added CORS support for OPTIONS preflight requests

Updates since last revision

Fix 1: Removed the 3-validator cap on genesis block creation:

• Removed the (addr_count < 3) ? addr_count : 3 cap on genesis_validator_count - now uses all participating validators
• Fixed the membership entry loop that was limited to i < 3 - now adds ALL collected signatures
• Updated error message to show actual validator count instead of hardcoded "3"

Fix 2: Added 30-second member collection wait period before locking genesis:

• Previously, the member list was locked immediately when 3 validators were discovered, preventing additional validators from joining
• Now waits up to 30 seconds (MXD_GENESIS_MEMBER_COLLECTION_WAIT_MS) for more validators to join after quorum is reached
• Locks immediately if 10 validators (maximum) are discovered before the timeout
• Logs progress every 5 seconds during the wait period

Fix 3: Fixed MXD_NETWORK_MAGIC environment variable parsing:

• Previously, the env var was loaded as raw bytes via mxd_load_secret_from_env(), which didn't work for hex strings
• Now properly parses hex strings (e.g., "0xDEADBEEF" or "DEADBEEF") using strtoul() with auto-detect base
• Logs the parsed network magic value for debugging
• This enables network isolation during testing by using a custom network magic

These changes ensure that when 10 nodes participate in genesis, all 10 will be included in the rapid stake table and receive 100 MXD each.

Review & Testing Checklist for Human

• Coinbase transactions in genesis: During testing, total_supply was 0 in the genesis block despite code changes. Verify coinbase transactions are actually being created and UTXOs are being stored correctly.
• Genesis timing impact: The 30-second wait period delays genesis block creation. Verify this is acceptable for your deployment scenarios and doesn't cause timeouts elsewhere.
• Network magic parsing edge cases: The strtoul() call doesn't validate input - invalid strings will silently return 0. Consider if this needs error handling.
• Wait loop behavior: The function returns early during the wait period and relies on being called repeatedly. Confirm the caller (consensus tick) handles this correctly.
• Security review of POST /transaction: The endpoint accepts private keys in the request body. This is intended for testing only and should NOT be used in production.
• Test plan: Deploy a 10-node testnet with MXD_NETWORK_MAGIC=0xDEADBEEF for isolation, observe the 30-second wait period in logs, verify genesis block gives all 10 validators 100 MXD (check UTXO balances via /balance/{address}), confirm rapid stake table shows 10 validators, then test transaction submission between nodes via the HTTP API.

Notes

• Transactions are added to local mempool only; propagation to peers relies on existing mempool sync mechanisms
• The API uses Ed25519 (algo_id=1) by default if not specified
• Amount is specified in MXD and converted to base units (×10^8)
• The minimum quorum requirement (3 validators) is still enforced; this change only removes the maximum cap and adds a wait period
• HTTP API must run on a different port than the monitoring system (e.g., 8081) to avoid port conflicts

Link to Devin run: https://app.devin.ai/sessions/dddc2bc4662d4c4a9bc984532b259c36
Requested by: @AlanRuno

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