Write-ahead log batching

A single-node OLTP engine's commit path is fsync-bound: every committing transaction calls fsync on the WAL to be durable, and under 50k tx/sec the disk can't sustain one fsync per commit, throughput collapses and commit latency spikes. You must raise commit throughput without weakening the durability guarantee (a transaction reported committed must survive a power loss). Design the WAL write/commit path: how you batch durability work across concurrent committers, how you keep latency bounded, and the knobs that trade latency for throughput.