feat: add exact equality for Jaccard sketches

[hawkingrei]

What changed

• Add exactly_equal to ThetaJaccardSimilarity and TupleJaccardSimilarity.
• Reuse the key-only union path to compare retained hashes and theta.
• Keep Tuple summary values and types outside the equality comparison.
• Cover empty, mutable/compact, seed mismatch, summary-independent, and zero-retained cases.

Why

This is a focused follow-up to #142 and its final review feedback. It adds only exact sketch-state equality; similarity_test and dissimilarity_test remain separate future work.

The zero-retained case intentionally distinguishes the two APIs: compute preserves an uncertain Jaccard interval, while exactly_equal compares the observable sketch state (retained hashes and theta), matching the other DataSketches implementations.

Related

• Follow-up to feat(theta): add jaccard similarity #142

Testing

• cargo x prepare-testdata
• cargo x check
• cargo x test
• cargo x lint

[ariesdevil]

rest LGTM

[ariesdevil]

LGTM