[ananas-block]

Prepare groth16-solana 0.3.0 and upgrade its direct Arkworks dependencies to 0.6. The minor release reflects breaking public API changes since 0.2.0, including verifying-key field changes and removed error variants and trait implementations.

Keep the optional Circom conversion API working by reading circom-prover's integer coordinates into validated Arkworks 0.6 points. Remove the direct ark-groth16 dependency, which this conversion no longer needs. Upstream circom-prover and the host Solana BN254 implementation still bring in Arkworks 0.5 transitively.

Add six conversion tests covering fixed output bytes, infinity, invalid coordinates, and points off the curve. Regenerate benchmarks and sync the README: standard Groth16 costs are unchanged; BSB22 costs increase by 2 CU per proof.

Validation passed:

• Full workspace tests, including two million differential cases against gnark and the Circom proof-generation test.
• All-feature crate tests, including the new conversion tests and verifying-key codegen tests.
• All seven feature combinations and the Rust 1.90 all-feature compile check.
• Solana SBF build, both no_std compile checks, and all eight CU benchmark cases.
• Workspace Clippy, all-feature crate Clippy, formatting, README sync, and diff checks.

Host checks used the installed macOS 26.5 SDK to avoid a local linker incompatibility with the macOS 27 SDK.