Expose jagged assist claim helper

[hero78119]

Problem

Downstream jagged PCS provers need the assist sumcheck claimed sum in the transcript before running the assist proof. Without an exported helper, callers either recompute the same ROBP-derived value separately or duplicate private jagged internals, which is fragile for GPU/opening integrations.

Approach

Expose assist_sumcheck_prove_and_append_claim so the assist claim is computed, appended, and proved from the same precompute path. Preserve the existing assist_sumcheck_prove API, and keep IOPProverState::claimed_sum() available for frontload sumcheck callers after the first round drops eval(0) from the proof message.

Testing

Validated through downstream integration checks:

• cargo check -p cuda_hal --features bb31
• cargo check --features gpu -p ceno_zkvm
• cargo check -p openvm-reth-benchmark --features gpu

Risk / Rollout

Low protocol risk: the verifier transcript order is unchanged; this only centralizes how the prover computes and appends the existing assist claim. Rollback is to keep callers computing the claim externally and call the original assist prover API.

Copilot Reviewer Directive (keep this section)

When Copilot reviews this PR, apply .github/copilot-instructions.md strictly.

• Perform review only; do not commit/push/propose code changes.
• Prefer inline comments on changed lines for each actionable finding.
• If inline comments are unavailable, use [severity] path:line (symbol) format.
• Prioritize soundness, performance, and architecture risks over style.
• Output order: findings by severity, then open questions, then brief summary.
• If PR description is empty or missing key context, report PR metadata: description as a finding.