Add explicit R1CS witness blinding API

[Sai (sai-deng)]

Summary

• preserve the existing safe witness APIs, which sample fresh blinds internally
• add R1CSWitnessBlind as a typed witness-commitment blind
• add _with_blind variants on the existing NovaWitness trait and R1CSWitness type for deterministic protocols
• keep ordinary Nova, Neutron, Spartan, tests, and fixtures on the safe random-default path
• release as nova-snark 0.74.0

Why

Normal callers should not need to manage commitment randomness. The typed explicit path makes that exceptional ownership visible without passing bare scalars or adding a second trait.

API

// Safe default: samples internally; returned witness exposes r_W().
cs.r1cs_instance_and_witness(&shape, &ck)?;

// Deterministic protocol/replay path.
let blind = R1CSWitnessBlind::from_scalar(derived);
cs.r1cs_instance_and_witness_with_blind(&shape, &ck, blind)?;

Validation

• default and experimental target compilation
• strict Clippy
• safe-default witness test
• typed-blind deterministic commitment test

[Wilson Nguyen (MercysJest)]

LGTM!

[Srinath Setty (srinathsetty)]

LGTM, left one minor nit.