Hack-Pair 0.2.7

Server startup race condition fixed — the exit event can fire before the data event on Windows (pipe buffering). Now:

• stdoutBuffer accumulates all output — seenStartup stays true once "running at" appears, even if the exit event fires first
• exit handler only rejects on non-zero exit codes when startup wasn't seen — code 0 with startup data waits for the health poll
• If server exits with code 0 but startup was seen, the health endpoint is still polled and startup resolves on timeout as a fallback
  Missing Windows env vars — added APPDATA, SystemRoot, TEMP, TMP to childEnv (some Node.js modules need these on Windows)
  Stale server cleanup — killExistingServer() kills any lingering process before spawning a new one. Server is also cleaned up in the catch block when room creation fails, so a failed attempt doesn't leave a zombie process blocking port 3001