[UTXO-BUG] make genesis rollback atomic and re-run safe

[createkr]

Summary

This PR fixes a UTXO migration safety bug where rollback_genesis() could run without an explicit transaction boundary, making partial deletion state possible if an error occurred between the rollback steps.

Root cause

rollback_genesis() performed multiple destructive DELETE operations without wrapping them in a single BEGIN IMMEDIATE / COMMIT block and without an explicit ROLLBACK on failure. That made rollback non-atomic and undermined re-run safety for genesis migration.

What changed

• Wrapped rollback_genesis() in a single BEGIN IMMEDIATE transaction
• Added explicit ROLLBACK handling on error
• Switched to timeout-aware SQLite connection setup for rollback
• Kept the fix tightly scoped to the rollback path
• Added focused regression tests covering:
  • migration creates genesis state
  • rollback removes all genesis data
  • rollback idempotency on empty state
  • re-run safety after rollback
  • no partial state after rollback
  • connection mode consistency

Validation

Added node/test_rollback_atomicity.py with 6 focused tests for rollback atomicity and re-run safety.

Scope

• node/utxo_genesis_migration.py
• node/test_rollback_atomicity.py

Payout Wallet

RTC1d48d848a5aa5ecf2c5f01aa5fb64837daaf2f35

[Scottcjn]

Merged. 100 RTC — High (Bounty #2819). Non-atomic rollback could corrupt state across nodes. Good defensive fix.