Checkpoint: the ledger, proven

Copy your Engine/InMemory/JsonlFile/replay and LedgerService into Solution, then add: 1. static String report(LedgerService svc, java.util.List<String> keys, java.util.concurrent.Executor exec) — per key, supplyAsync(() -> key + "=" + svc.get(key), exec) with orTimeout(50, MILLISECONDS) and handle → key + "=down" on failure; allOf(...).orTimeout(500, MILLISECONDS).join(); join parts in submission order, joined with ",". 2. static String prove(String seed) — the proof: build a JsonlFile over a fresh temp file, record seed/a=1, seed/b=2 through a LedgerService; build an InMemory engine, record the SAME operations through another LedgerService; run report( ..., List.of("a","b"), sameExecutor) on both; return the two report strings joined with "|" — they must be IDENTICAL strings for the test to pass. 3. static boolean survivesRestart(java.nio.file.Path file) — engine over file, record x=9; build a SECOND service via replay(file); return whether the replayed service get("x") equals "9".