add fee-aware mining, coinbase validation, and reorg-safe orphan handling

Mining: blocks now include mempool txs, select spendable txs by fee, and pay coinbase as base reward + fees in main.c.
 - Consensus: block validation now enforces coinbase accounting and rejects invalid coinbase placement, including coinbase on amount2, in block.c and transaction.c.
 - Chain state: rollback now rebuilds currentSupply/currentReward, and block addition preflights spendability before mutating balances in chain.c.
 - Orphans/reorgs: orphan retry is safer, rollback-triggered sync reattaches orphans immediately, and transient orphan failures no longer drop blocks in orphan_pool.c and main.c.
 - Networking/mempool: node lifecycle now initializes the mempool, broadcasts can exclude one peer, and mempool snapshotting supports mining selection in net_node.c and txmempool.c.
 - Ledger simulation: added non-mutating spendable-transaction selection for block assembly in balance_sheet.c.

include/txmempool.h

@@ -13,6 +13,7 @@ void TxMempool_Init();
 // Assumed that the transation was confirmed to be valid
 int TxMempool_Insert(signed_transaction_t tx);
 bool TxMempool_Lookup(uint8_t* txHash, signed_transaction_t* out);
+bool TxMempool_Snapshot(signed_transaction_t** outTxs, size_t* outCount);
 void TxMempool_Print();
 void TxMempool_Destroy();