fix: reject duplicate private function selectors

[AztecBot]

Summary

Private AuthWit action hashes bind a private function by selector, while private function membership is committed as (selector, vkHash). This change rejects duplicate private selectors during artifact-to-class construction and private function tree construction so a class cannot commit two private functions that share the AuthWit selector identity.

Fix

• Add a duplicate-selector check for private functions before building the private function tree/root.
• Reuse that check in getContractClassFromArtifact and include colliding artifact function names in the error.
• Add regressions for the known colliding selector pair and for direct private-function root construction.

Testing

• git diff --check
• awk 'length($0)>120 { print FILENAME ":" FNR ":" length($0) ":" $0 }' yarn-project/stdlib/src/contract/contract_class.ts yarn-project/stdlib/src/contract/contract_class.test.ts yarn-project/stdlib/src/contract/private_function.ts yarn-project/stdlib/src/contract/private_function.test.ts
• yarn workspace @aztec/stdlib test src/contract/contract_class.test.ts src/contract/private_function.test.ts (blocked before tests: missing node_modules state)
• yarn install --immutable (blocked: @aztec/noir-acvm_js@portal:../noir/packages/acvm_js manifest not found)

Issues

• Closes AztecProtocol/aztec-claude#912 — private AuthWit action hash aliases duplicate private selectors

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Created by claudebox · group: kernels-audit-remediation