[UTXO-BUG] reject negative fees in transfer validation

[createkr]

Summary

This PR fixes a UTXO accounting bug where a negative fee_nrtc value could weaken the conservation check in apply_transaction() and allow a normal transfer to create more value in outputs than existed in inputs.

Root cause

apply_transaction() enforced value conservation using output_total + fee <= input_total, but it did not validate that fee_nrtc was non-negative. A caller could set a negative fee to offset excess outputs and bypass the conservation check.

What changed

• Rejected transactions with negative fee_nrtc in apply_transaction()
• Added a focused regression test proving a negative-fee transfer is rejected and balances remain unchanged

Why this matters

This affects normal transfer transactions with real inputs, not just minting-style paths. Without the non-negative fee guard, a transfer can inflate total supply by encoding the excess value as a negative fee.

Scope

• node/utxo_db.py
• node/test_utxo_db.py

Payout Wallet

RTC1d48d848a5aa5ecf2c5f01aa5fb64837daaf2f35

[Scottcjn]

Merged. 200 RTC — Critical (Bounty #2819). Negative fee_nrtc bypasses conservation check — money printer exploit. Solid find.