gpu: add device-backed WITGEN commit path

[hero78119]

Problem

GPU_WITGEN,CACHE=1 can produce witness traces on GPU, but the prover path still needs a clean device-resident commit flow. The goal is to keep GPU-generated witness data usable through commit without falling back to replay/deferred raw-cache logic or unnecessary host materialization.

Design Rationale

This PR treats GPU witness output as the source of truth for the commit path: traces are normalized into device-backed row-major metadata, committed through the GPU PCS path, and released once q'/commit no longer needs the raw backing. The post-commit proving flow stays aligned with the existing CPU_WITGEN path so correctness-sensitive transcript, opening, and proof assembly logic remain shared.

The design avoids retaining replay plans as a second witness source. This keeps ownership simpler: GPU witness generation owns raw device buffers until q'/commit construction, then releases them before chip proving pressure grows.

Change Highlights

• ceno_zkvm: add GPU witness/device-backed trace commit path for GPU_WITGEN,CACHE=1.
• ceno_zkvm: keep post-commit proving and opening flow shared with the existing GPU prover path.
• ceno_zkvm: release shard GPU witness caches after proof construction.
• gkr_iop: support GPU-side batched main-constraint proving integration.

CI Benchmark Summary

Compared CI benchmark runs:

• GPU_WITGEN: original PR benchmark numbers, kept for context.
• CPU_WITGEN: 26067686212, branch feat/witgen_gpu, CENO_GPU_ENABLE_WITGEN=0
• CPU_WITGEN (baseline): 26037135648, branch feat/update_dep, CENO_GPU_ENABLE_WITGEN=0

Metric	GPU_WITGEN	CPU_WITGEN	CPU_WITGEN (baseline)	Notes
reth-block E2E	111s	80.2s	83.2s	CPU_WITGEN feature branch is fastest
app.prove	107s	65.6s	68.2s	CPU_WITGEN feature branch improves 2.6s vs baseline
app_prove.inner	96.6s	65.6s	68.2s	Same trend as app.prove
Witness total	35.43s	40.85s	39.84s	GPU_WITGEN remains faster raw witness gen
Proof total	60.70s	62.15s	64.78s	CPU_WITGEN feature branch improves proof total vs baseline
commit_traces total	12.35s	17.410s	17.450s	GPU_WITGEN commit path remains faster
commit_traces avg/shard	950ms	n/a	n/a	Original GPU_WITGEN per-shard metric kept
prove_tower_relation_gpu total	n/a	119.624s	22.515s	Nested/overlapped span increased in feature run
prove_batched_main_constraints total	n/a	7.934s	7.639s	Slight CPU_WITGEN regression
pcs_opening total	9.91s	9.857s	10.061s	Stable
q commit total	8.25s device_q	n/a	n/a	Original GPU_WITGEN q metric kept
q commit avg/shard	634ms	n/a	n/a	Original GPU_WITGEN q metric kept
q inner commit avg	449ms	n/a	n/a	Original GPU_WITGEN q metric kept
CPU/GPU overlap gap	n/a	3.170s	3.200s	CPU_WITGEN overlap unchanged
Overall result	111s	80.2s	83.2s	CPU_WITGEN feature branch beats baseline; GPU_WITGEN still loses overall due to lost overlap

Conclusion	Evidence
GPU_WITGEN still improves commit/witness subpaths	Original GPU_WITGEN has faster witness total and commit_traces than CPU_WITGEN
GPU_WITGEN still loses overall	111s E2E vs 80.2s CPU_WITGEN due to lost shard witness/proof overlap
CPU_WITGEN feature branch is slightly faster than CPU_WITGEN baseline	reth-block improves by 3.0s; app.prove improves by 2.6s
Commit/opening path is stable for CPU_WITGEN	commit_traces and pcs_opening are within ~0.2s across CPU runs

Benchmark / Performance Impact

This is performance-sensitive. CI benchmark runs are used for comparable end-to-end numbers because local wall time depends heavily on runner scheduling and GPU availability.

Operation

Operation	master (s)	this PR (s)	Improve (master -> this PR)
Reth proving benchmark	See benchmark CI	See benchmark CI	See benchmark CI

Layer

Layer	master (s)	this PR (s)	Improve (master -> this PR)
Witness commit/q' path	Host/materialized path	Device-backed GPU path	Reduces host materialization and extra copies
Post-commit proving	Existing GPU flow	Existing GPU flow	Intended to remain unchanged

Benchmark command(s):

# ceno-reth-benchmark CI, GPU_WITGEN,CACHE=1 and CPU_WITGEN,CACHE=1 comparison runs

Environment (CPU/GPU, core count, rust toolchain, commit hash):

CI benchmark runner metadata and commit hashes are recorded in the linked workflow runs.

raw data:

• master: benchmark CI artifacts
• this PR: benchmark CI artifacts

Testing

cargo fmt --check
cargo check -p ceno_zkvm --features 'gpu,u16limb_circuit' --config 'patch."https://github.com/scroll-tech/ceno-gpu-mock.git".cuda_hal.path="../ceno-gpu/cuda_hal"'

Risks and Rollout

• Main risk is lifetime/ownership mistakes around device-backed witness buffers; the rollout keeps release points explicit and avoids replay cache ownership.
• If regressions appear, disable CENO_GPU_ENABLE_WITGEN to return to the existing CPU_WITGEN GPU proving path.

Follow-ups (optional)

• Continue profiling per-chip GPU witness generation and q' construction.
• Add scheduler-level overlap once device memory booking is precise enough.

Copilot Reviewer Directive (keep this section)

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

[hero78119]

this is some debug left-over. Fixed this bring back concurrent prove and baseline improve 1.037x e2e performance