Stage 2: The Write-Ahead Log

RAM dies with the process. Before a real engine touches the memtable, it appends the operation to a write-ahead log (WAL) — an append-only record that survives crashes. On restart, replaying the log rebuilds the memtable exactly. This is the durability contract of every serious database.

Extend your store: createStore(existingLog) where:

• every set appends { op: "set", key, value } to a store.log array, every del appends { op: "del", key }
• store.log is the array itself (the harness inspects it)
• when createStore(existingLog) receives a previous run's log, it replays it so the rebuilt store answers gets identically

The crash test below is literal: it builds a store, "crashes" it, and hands your constructor the old log. Your engine either remembers or it doesn't.