fix: throw NonZeroExitError when process is killed by signal - #146

Summary

When a child process is killed by a signal (SIGABRT, SIGKILL, SIGTERM, etc.), throwOnError: true silently resolves instead of throwing NonZeroExitError.

Root cause: When a process dies by signal, Node.js sets child.exitCode = null and child.signalCode to the signal name. The exitCode getter returns undefined for null, and the throwOnError check excludes undefined — so signal-killed processes slip through. Same flaw exists in xSync.

Changes

• Add signalCode to CommonOutputApi interface and ExecProcess getter
• Fix throwOnError in _waitForOutput and async iterator to throw when signalCode != null
• Fix xSync throwOnError to also check signalCode
• Update NonZeroExitError message to "Process was killed with signal SIGTERM" for signal deaths; expose signalCode getter
• Add regression tests for async and sync signal-kill paths

Fixes #144