Validate the hierarchical estimation approach for XAttention: - Test 1: Math equivalence (diff = 0.0) between hierarchical and direct - Test 2: Score + threshold selection strategy (replaces mask + voting) - Test 3: Performance benchmark (41x speedup) Uses pure torch + xattn kernels, independent of nanovllm framework. Generated with [Claude Code](https://claude.ai/code) via [Happy](https://happy.engineering) Co-Authored-By: Claude <noreply@anthropic.com> Co-Authored-By: Happy <yesreply@happy.engineering>