ScalarField support for secp256r1 (P-256)

secp256k1 and bn254 have ScalarField implementations in halo2-base, but secp256r1 (P-256) doesn't.

P-256 is used in ES256 signatures (OAuth/JWT) and WebAuthn. The field types already have all required traits - just needs the macro invocation.

Would be a 2-line addition following the existing pattern.