Every sign/verify/key-derive site built a fresh context via
Secp256k1::new(), which allocates a Secp256k1<All> and runs
randomization/blinding table setup on each call. Introduce one
crate-wide LazyLock<Secp256k1<All>> and reuse it across the local, peer,
and auth sites (and their tests). Behavior-neutral: identical secp256k1
API calls, only the context lifetime changes, and the shared All context
still performs the standard construction-time blinding.