feat(theta): add jaccard similarity

[hawkingrei]

Summary

• Add ThetaJaccardSimilarity for Theta sketches.
• Return lower bound, estimate, and upper bound for the Jaccard index.
• Port core coverage from the C++ theta_jaccard_similarity_test.cpp cases.

Motivation

This closes one small P0 parity gap with apache/datasketches-cpp: Rust had Theta sketching and intersection support, but no Jaccard similarity API.

Implementation Notes

• Reuses existing ThetaIntersection for the intersection side.
• Builds the two-input union locally inside the Jaccard implementation without exposing a full ThetaUnion API in this PR.
• Ports the C++ sampled-ratio bound approximation locally for Jaccard bounds.

Tests

• cargo check -p datasketches --features theta
• cargo test -p datasketches --features theta --test theta_jaccard_similarity_test
• cargo test -p datasketches --features theta --lib --test theta_intersection_test --test theta_jaccard_similarity_test --test theta_sketch_test

Full cargo test -p datasketches --features theta currently reaches unrelated failures in theta_serialization_test because local datasketches/tests/serialization_test_data/... files are missing.

[ZENOTME]

Looks like we better have union implementation here first.

[ZENOTME]

These are general math function which can be place in bounds_binomial_proportions.rs

[ZENOTME]

We also miss exactly_equal but it's ok to postpone it at next PR.