chore: slim down world-id-primitives deps

[paolodamico]

Feature flags proof dependencies in the primitives crate to reduce the amount of dependencies pulled in upstream crates.

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Note

Medium Risk
Medium risk due to API signature changes in world-id-primitives (error type switch to PrimitiveError) and new feature-flagged compilation paths that could break downstream builds if features are misconfigured.

Overview
Slims world-id-primitives dependency surface by introducing a proofs feature that gates Groth16/circuit-input related code and dependencies (including circuit_inputs and ZeroKnowledgeProof::from_groth16_proof). Downstream crates that need proof generation (authenticator, proof, oprf-node, oprf-dev-client) now enable world-id-primitives with features = ["proofs"].

Modernizes signing deps and error handling by switching request signatures and the internal Signer to use alloy-signer/alloy-signer-local instead of alloy re-exports, and by changing Credential::{claims_hash,hash,sign,verify_signature} to return PrimitiveError (with callers updated accordingly). Minor doc-link cleanup in authenticator docs accompanies the refactor.

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[paolodamico]

@codex review

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