fix: keep block save and tx confirmation in one transaction

[createkr]

Makes BlockProducer.save_block() and TransactionPool.confirm_transaction() share a single SQLite connection so that block insertion and all transaction confirmations are a single atomic unit.

Problem

save_block() opened its own DB connection and called confirm_transaction() for each transaction in the block. confirm_transaction() opened a separate connection and committed independently. This meant:

• A crash mid-save could leave a block saved with partially-confirmed transactions
• Failed confirmations were silently ignored — the block was saved anyway
• No way to roll back the entire block if any transaction confirmation failed

Fix

• rustchain_tx_handler.py: Added optional conn parameter to confirm_transaction(). When provided, the method uses the caller's connection and does not manage its own transaction boundary. The core logic is extracted into _do_confirm(cursor) to avoid duplication.
• rustchain_block_producer.py: save_block() now passes its connection to each confirm_transaction() call. If any confirmation returns False, the entire block save is aborted and rolled back.

Changes

• node/rustchain_tx_handler.py — confirm_transaction() accepts optional conn parameter; extracted _do_confirm() inner function
• node/rustchain_block_producer.py — save_block() passes conn=conn to confirm_transaction() and aborts on failure
• node/tests/test_f10_block_save_atomicity.py — 5 new tests:
  • test_confirm_with_shared_connection_succeeds
  • test_confirm_rollback_on_shared_connection
  • test_confirm_fails_insufficient_balance_shared_conn
  • test_standalone_confirm_still_works (backward compat)
  • test_multi_tx_atomic_on_shared_conn

Backward Compatibility

The conn parameter is optional with a default of None. Existing callers that don't pass a connection continue to use the standalone path with their own transaction management. No API breakage.

[createkr]

For bounty payout, please use RTC wallet: RTC1d48d848a5aa5ecf2c5f01aa5fb64837daaf2f35.

[Scottcjn]

✅ Merged! 75 RTC awarded for atomicity fix — block save.

Payment will be sent to wallet RTC1d48d848a5aa5ecf2c5f01aa5fb64837daa as requested.

Total this batch: 200 RTC. Thank you @createkr — your 3 fixes today touched security, data integrity, and epoch correctness. This is core contributor work.