feat: add cosine and dice similarity metrics

[alok-108]

Related to #124

This PR adds two similarity metrics to the existing similarity module:

• Cosine similarity (cosine)
• Dice / Sørensen-Dice similarity (dice)

Changes:

• Implemented cosine() and dice() in selector/measures/similarity.py using NumPy.
• Exposed them through __all__ where appropriate.
• Extended pairwise_similarity_bit() metric dispatch to support "cosine" and "dice".
• Added focused unit tests for numerical correctness, shape mismatch, invalid dimensionality, zero-vector behavior, and pairwise matrix properties.
• Added concise documentation for the new similarity metrics and their relationship to corresponding distance forms.

Notes for reviewers:

• No new dependencies.
• Backward compatible.
• No unrelated refactoring.
• Zero-vector behavior is explicitly defined and tested.
• Dice documentation distinguishes the continuous vector formulation from SciPy's boolean-specific dice distance.