TxGuard: Zcash v5 Consensus Branch‑ID Preflight (CLI + GitHub Action)

### Terms and Conditions - [x] I agree to the [Grant Agreement](https://9ba4718c-5c73-47c3-a024-4fc4e5278803.usrfiles.com/ugd/9ba471_f81ef4e4b5f040038350270590eb2e42.pdf) terms if funded - [x] I agree to [Provide KYC information](https://9ba4718c-5c73-47c3-a024-4fc4e5278803.usrfi