test: add penalty transaction test

[ABallarini]

Summary

Adds a deterministic integration test (src/test/penalty_transaction.rs) that verifies protocol penalty behavior when a node broadcasts a revoked commitment state (per BOLT 3 / BOLT 5).

High-Level Overview

1. Setup & Channel Creation: Node A issues an RGB asset and opens a channel with Node B.
2. State Transition: A sends a keysend payment to B (moving to State 2 and revoking State 1).
3. Breach Simulation: Node A is rewound to a backup of State 1 and issues a forced channel closure on-chain.
4. Justice & Consolidation: Node B detects the counterparty breach via ldk.log, constructs and mines a justice (penalty) transaction and processes the output consolidation sweep.
5. Structural & Fee-Invariant Verification: Asserts exact spendable BTC balances (node2_btc_before + sweep_out) using structural transaction checks (1-in/1-out) and a bounded fee-sensitivity invariant (< 5_000 sats total fees).
6. RGB Asset Verification & Known Issue: Asserts post-sweep spendable RGB asset balances on both nodes.

---

⚠️ ISSUE

• RGB Asset Sweep Behavior (Assert RGB asset balances):
  There is currently an issue in the RGB asset assertion phase. While Node A correctly retains its uncommitted 400 RGB units (ISSUE_AMT - RGB_ON_NODE_A), the honest node (Node B) does not currently receive the 600 RGB units carried by the revoked to_local output upon sweeping but are burnt. The test framework currently flags this discrepancy during the asset balance assertion step.

📖 Specification & Step-by-Step Flow: View HackMD Specification