When the studio process is killed (SIGTERM/SIGKILL), atexit handlers
may not run in the subprocess orchestrator, leaving llama-server
processes orphaned and holding GPU memory. This caused OOM errors when
trying to load a new model after a studio restart.
On init, LlamaCppBackend now runs pgrep to find and SIGKILL any stale
llama-server processes before starting fresh.
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