Add DAG-based variant of BasisTranslator transpiler pass and opaque EquivalenceLibrary type in C Api

[bruno-ah-um]

Continuation of #15783 and #16261

@eliarbel: please review

Summary

Adds qk_transpiler_pass_basis_translator(), the DAG-based variant of the BasisTranslator standalone transpiler pass in the C API. This function operates directly on a QkDag instead of a QkCircuit, avoiding circuit↔DAG round-trip conversions when chaining multiple passes together.

Introduces QkEquivalenceLibrary, a new opaque C type that wraps Qiskit's standard gate equivalence library. qk_transpiler_pass_basis_translator() accepts the equivalence library as an explicit input parameter. This PR implements only qk_equivalence_library_new_standard() and qk_equivalence_library_free().

The key improvement over the standalone variant (qk_transpiler_pass_standalone_basis_translator) is that callers can construct the equivalence library once and reuse it across repeated pass invocations, avoiding the cost of regenerating it on every call.

Tests

4 new C tests in test/c/test_basis_translator.c:

• test_dag_in_basis: DAG already in the target basis is left unchanged
• test_basic_dag_basis_translator: H gate translated to U gate basis
• test_toffoli_dag_basis_translator: CCX decomposed into {H, T, Tdg, CX} (15 gates total)
• test_dag_basis_translator_reused_library: Run the pass twice on different DAGs sharing the same equivalence library

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description: Claude Sonnet 4.6
• I used the following tool to generate or modify code: Review code, improve it Sonnet 4.6

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