Add CircleCI semver-compatibility check job for release branches

[Copilot]

This PR introduces a CI gate for API/backward-compatibility regressions in snarkOS, aligned with the existing snarkVM semver-check pattern. It adds a dedicated semver check script and integrates it into CircleCI’s merge-focused workflow branches.

• Semver check script

  • Added .circleci/semver-checks.sh.
  • Installs cargo-semver-checks@0.43.0, ensures baseline commit visibility in shallow clones, and runs workspace semver checks against a pinned baseline revision (v4.6.4 commit).

• New CircleCI job

  • Added check-cargo-semver-checks in .circleci/config.yml.
  • Uses full checkout (method: full) and existing setup/cleanup commands to execute the new script.

• Workflow integration

  • Wired check-cargo-semver-checks into merge-devnet-workflow for:
    • canary
    • testnet
    • mainnet
    • staging

# .circleci/semver-checks.sh
cargo install cargo-semver-checks@0.43.0 --locked
git fetch --unshallow || true
cargo semver-checks --workspace --default-features --baseline-rev $BASELINE_REV