Similarity

[lisette-espin]

implement equation 8 from LLMScholarBench

[manzannita]

Implemented in compute_similarity / aggregate_similarity (code/libs/metrics/aggregators.py), following Eq. 8 of LLMScholarBench. Shipped in bacb479; covered by tests/test_similarity_pipeline.py and tests/test_structural_metrics.py (38 tests pass).