Consider providing block result to hook lambda

Biggest problem is that we don't get signal information on the JVM aside from just being able to infer we were signaled.

Maybe
```kotlin
sealed interface BlockResult {
data object Success : BlockResult
data class Failure(val t: Throwable) : BlockResult {
val isCancellation: Boolean get() = t is CancellationException
}
data class Signalled(val signal: Int?) : BlockResult
}
```

where the JVM gets a null signal value, and Windows gets only `SIGINT`?

Start by tracing the hook lambda API and the platform-specific JVM and Windows signal handling mentioned in the issue. Define how the proposed BlockResult variants and platform-specific signal values should be exposed, then verify success, failure, cancellation, and signalling behavior across those platforms.

Written by the indexing model from the issue text.