Simplify Nova witness blind API

[Srinath Setty (srinathsetty)]

Closes #511

[Copilot]

🟡 Changes recommended

The new API removes the explicit-blind pathway and changes the existing method signature in a way that appears to conflict with the direction described in issue #511, leaving deterministic/replayable callers without a direct explicit-blind option.

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Pull request overview

This PR updates Nova’s R1CS witness-extraction flow to make witness blinding come from a caller-supplied RNG, and adjusts internal call sites/tests accordingly.

Changes:

• Changes the NovaWitness::r1cs_instance_and_witness API to accept rng: &mut R and wires OsRng through existing call sites.
• Updates R1CSWitness::new to sample its blinding factor from a provided RNG (and updates determinism tests to use seeded RNGs).
• Bumps crate version to 0.76.0.

File summaries

File	Description
src/spartan/direct.rs	Passes &mut OsRng into witness extraction during direct proving.
src/r1cs/mod.rs	Switches witness blinding to RNG-based sampling; updates determinism test accordingly.
src/nova/nifs.rs	Updates tests/bench paths to supply &mut OsRng for witness extraction/construction.
src/nova/mod.rs	Threads &mut OsRng into base-case witness extraction.
src/nova/circuit/mod.rs	Updates circuit tests to pass &mut OsRng into witness extraction.
src/neutron/relation.rs	Updates test to pass &mut OsRng into witness extraction.
src/neutron/nifs.rs	Updates tests/bench paths to pass &mut OsRng into witness extraction/construction.
src/neutron/mod.rs	Threads &mut OsRng into witness extraction in the main Neutron flow.
src/neutron/circuit/mod.rs	Updates circuit tests to pass &mut OsRng into witness extraction.
src/gadgets/ecc.rs	Updates gadget tests to pass &mut OsRng into witness extraction.
src/frontend/r1cs.rs	Changes the NovaWitness trait method signature to require an RNG and removes the explicit-blind path.
src/frontend/mod.rs	Updates frontend tests to pass &mut OsRng into witness extraction.
Cargo.toml	Version bump to 0.76.0.

Review details

• Files reviewed: 13/13 changed files
• Comments generated: 2
• Review effort level: Lite

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[Wilson Nguyen (MercysJest)]

LGTM!

[Abhiram Kothapalli (abhiramkothapalli)]

LGTM!