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.

TODO.txt

@@ -17,6 +17,11 @@ A loophole in the reorg penalty system could potentially exist where someone bro
 TO TEST:
 Implement Horizen's "Reorg Penalty" system to make it harder for the young chain to be attacked by a powerful miner.
 
+NOTE:
+Because tx sizes are currently fixed, mining can use raw fee ordering for now. If tx sizes ever become dynamic, revisit selection to consider fee/byte instead.
+
+Mempool snapshotting for mining should hold the lock only long enough to copy pending txs, but if the mempool grows very large that copy may still be non-trivial.
+
 DONE:
 I want to move away from the Monero emission. I want to do something a bit radical for cryptocurrency, but I feel like it's necessary to make it more like money:
 a constant inflation rate of 1.5% per year. It's lower than fiat (USD is ~2.8% per year), and it additionally doesn't fluctuate during crisis. It's constant.