chore: mark arkworks wrappers repr(transparent)

[0xAndoroid]

Summary

• Add #[repr(transparent)] to all single-field arkworks wrapper types so they share memory layout with the inner type.
• Enables safe transmute, FFI compatibility, and zero-cost slice-of-wrapper ↔ slice-of-inner conversions.

Changes

• src/backends/arkworks/ark_field.rs — ArkFr
• src/backends/arkworks/ark_group.rs — ArkG1, ArkG2, ArkGT
• src/backends/arkworks/ark_setup.rs — ArkworksProverSetup, ArkworksVerifierSetup

Multi-field structs (Blake2bTranscript, ArkworksPolynomial, PreparedCache) and unit structs (BN254, G1Routines, G2Routines) skipped — repr(transparent) requires exactly one non-zero-sized field.

Testing

CI covers this (build + clippy already verified locally).