Capstone: The Ledger API in Kotlin

./gradlew test --tests '*Acceptance*'
#  ✓ 100 concurrent identical POST /v1/transfers → exactly 1 transfer
#  ✓ Σ postings = 0 for every transfer (Kotest property, 1,000 cases)
#  ✓ payout timed out at the rail → UNKNOWN → only a rail confirmation settles it

// concurrency test with coroutines
@Test fun idempotent() = runBlocking {
    val key = UUID.randomUUID().toString()
    val ids = (1..100).map { async(Dispatchers.IO) { client.createTransfer(req, key).id } }.awaitAll().toSet()
    assertEquals(1, ids.size)
}

// payout polling: one failing payout does not cancel the rest
val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
pending.forEach { p -> scope.launch { withTimeout(15.minutes) { pollUntilFinal(p) } } }