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.
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via [Happy](https://happy.engineering)
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>
