Make ExitWaiter.doExit idempotent to avoid double-resuming continuations

[radheradhe01]

Problem

ExitWaiter.doExit resumes every stored continuation but never guards against being called twice and never clears the array:

public func doExit(exitStatus: ExitStatus) {
    for cc in continuations {
        cc.resume(returning: exitStatus)
    }
    self.exitStatus = exitStatus
}

doExit can be reached more than once for the same process. In the exec path, the registered onExit callback calls releaseWaiters (→ doExit) and then performs further work that can throw (process.delete()); ExitMonitor.track catches that error and invokes the same onExit again, calling doExit a second time. Resuming a CheckedContinuation twice is a fatal trap.

Fix

Make doExit idempotent: return early if the exit has already been recorded, set exitStatus first, and clear the continuations before resuming them so they cannot be resumed again.

Notes

All access is serialized on the owning actor; the guard simply makes the "exit fires once" invariant explicit and crash-safe.