feat: implement Z-X bialgebra rewrite rule

[dlyongemallo]

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[Copilot]

Pull request overview

Implements a Z–X bialgebra rewrite rule in the ZX-calculus core rules and adds a dedicated test suite to validate its correctness (including scalar factors and boundary handling).

Changes:

• Added check_bialg, bialg_unchecked, and the checked wrapper bialg implementing the Z–X bialgebra rule with appropriate structural and phase preconditions.
• Implemented the bialgebra transformation to remove the central Z/X spiders, connect their neighborhoods bipartitely with normal edges, and apply the sqrt(2)^((m-1)*(n-1)) scalar factor.
• Added a comprehensive set of unit tests covering basic, boundary, asymmetric, and non-matching cases for the bialgebra rule, verifying semantic preservation via to_tensor4().

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