zk-credential-verifier: wire up CI workflow pinned to a Protocol-25-capable Soroban toolchain

[Dannyswiss1]

Description

Add a CI workflow so every PR touching zk-credential-verifier/ is automatically built, tested, and linted, matching the CI coverage set up for the other LockA contract crates — with the added requirement of pinning a toolchain that actually exposes the BN254/Poseidon2 host functions this crate depends on.

Tasks

• Add (or extend the existing) GitHub Actions workflow to also run on PRs touching zk-credential-verifier/**
• Pin the Rust toolchain and soroban-sdk/stellar-cli versions to the ones confirmed by the research-spike issue as supporting Protocol 25 host functions
• Steps: install Rust toolchain + wasm32v1-none target, cargo build --target wasm32v1-none --release, cargo test, cargo clippy -- -D warnings, cargo fmt --check
• Reuse Cargo registry/build artifact caching already configured for the workspace
• Verify the workflow passes on a test PR before merging

Acceptance Criteria

• CI workflow runs automatically on PRs touching this crate, uses a pinned Protocol-25-capable toolchain, and fails the check if build, tests, clippy, or fmt fail.

Location

.github/workflows/ (workflow scoped to zk-credential-verifier/)