[follow up 1299]: fix gpu build && optimize witness build and avoid GPU OOM

[hero78119]

Problem

PR #1299 changed GKR output/eval wiring, but the GPU proving flow still treated build_main_witness as if only the old read/write/lookup outputs existed. That caused two issues:

• stale scheduler / memcheck estimates after refactor(rotation): move rotation proof to chip level and unify selec… #1299
• unnecessary GPU witness materialization before tower proving, which can push large Keccak payloads into OOM on 4090-class cards

Design Rationale

Keep the proof shape and verifier unchanged, and fix this entirely in prover-side staging:

• route first-layer GKR output groups with prover-only stage metadata
• materialize only tower-needed outputs before tower proving
• keep ECC / rotation self-contained in their existing submodules
• update GPU memory estimation to match the post-refactor(rotation): move rotation proof to chip level and unify selec… #1299 output topology

This reduces VRAM pressure during tower proving without changing proof semantics.

Change Highlights

• ceno_zkvm
  • add prover-only GkrOutputStageMask routing for first-layer output groups
  • build only tower-facing witness outputs before prove_tower_relation
  • keep ECC / rotation on their existing dedicated witness/eval paths
  • update GPU memory estimation for post-refactor(rotation): move rotation proof to chip level and unify selec… #1299 GKR outputs and tower-stage residency
  • update local precompile/test callsites to the new gkr_witness API
• gkr_iop
  • extend witness generation with filtered materialization APIs for CPU/GPU backends
  • keep filtering internal to prover execution; no verifier/proof-format changes

Benchmark / Performance Impact

Primary intent is memory reduction, not throughput optimization.

Operation

Benchmark command(s):

CENO_GPU_ENABLE_WITGEN=1 cargo run --config net.git-fetch-with-cli=true --features gpu --release --package ceno_zkvm --bin e2e -- --platform=ceno --max-cycle-per-shard=1600 examples/target/riscv32im-ceno-zkvm-elf/release/examples/keccak_syscall

Ceno reth
https://github.com/scroll-tech/ceno-reth-benchmark/actions/runs/24667766891

Testing

cargo make clippy
cargo check -p ceno_zkvm --features gpu
CENO_GPU_ENABLE_WITGEN=1 cargo run --config net.git-fetch-with-cli=true --features gpu --release --package ceno_zkvm --bin e2e -- --platform=ceno --max-cycle-per-shard=1600 examples/target/riscv32im-ceno-zkvm-elf/release/examples/keccak_syscall

Risks and Rollout

• Risk is limited to prover-side witness staging and GPU memory estimation.
• Verifier behavior and proof format are intentionally unchanged.
• If this regresses proving, rollback can revert the new output-stage routing and filtered witness materialization together.

Follow-ups (optional)

• Add a targeted large-payload regression check for post-refactor(rotation): move rotation proof to chip level and unify selec… #1299 GPU WITGEN memory peaks.

Copilot Reviewer Directive (keep this section)

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