Rewrote capture_graph_mega to use cudarc's begin_capture/end_capture
instead of raw cuda_sys. Same fix that resolved graph_aux hang.
All graph types (graph_mega, graph_aux, graph_spectral) now use
cudarc wrapper consistently. Falls back to split graphs if mega fails.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>