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

[hawkingrei]

Addressed by moving the general binomial proportion helpers into common::bounds_binomial_proportions; jaccard_similarity.rs now calls approximate_lower_bound_on_p / approximate_upper_bound_on_p from that common module.

[ZENOTME]

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

[tisonkun]

We must not have a unit struct ThetaJaccardSimilarity for creating JaccardSimilarity.

Let me consider the API and review the helper functions location.

Typically, here are a few API issues that should be resolved:

1. Should have JaccardSimilarity::between / JaccardSimilarity::between_with_seed, rather than ThetaJaccardSimilarity::jaccard / ThetaJaccardSimilarity::jaccard_with_seed.
2. JaccardSimilarity should expose its internal fields with getter, not pub fields, because the fields are immutatble.

[hawkingrei]

Addressed the API feedback from @tisonkun:

• Removed the ThetaJaccardSimilarity unit struct.
• Moved the public constructors to JaccardSimilarity::between(...) and JaccardSimilarity::between_with_seed(...).
• Made JaccardSimilarity fields private and exposed immutable getters: lower_bound(), estimate(), and upper_bound().

The tests have been updated to use the new API.