fix: segfault — painting finalizer was destroying runtime-owned buffers
JS_NewObjectClass(painting_class_id) for nopaint buffer/painting
meant GC would call fb_destroy() on runtime-owned framebuffers.
Now the finalizer skips buffers that match current_rt->nopaint_*.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>