perf: avoid cloning CPU witness MLEs

[hero78119]

Problem

left-over from #923. CPU trace commit cloned large witness MLEs before proving, adding avoidable memory traffic on the prover hot path.

Design Rationale

Keep committed witness MLEs behind Arc and drain/transport ownership where possible, avoiding deep clones without changing proof semantics.

Change Highlights

• ceno_zkvm: return Arc witness MLEs from trace commit and consume structural MLEs during transport.
• ceno_zkvm: keep GPU trait shape aligned while preserving existing GPU behavior.

Benchmark / Performance Impact

Operation

Operation	master (s)	this PR (s)	Improve (master -> this PR)
CPU proving, keccak e2e shard total	6.942	6.596	4.98% faster
GPU proving, keccak e2e shard total	1.191	1.186	0.44% faster

Layer

Layer	master (s)	this PR (s)	Improve (master -> this PR)
N/A: shard-level proving total measured	N/A	N/A	no regression observed

Benchmark command(s):

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

Environment: local x86_64 Linux, release build, local ../ceno-gpu/cuda_hal patch for GPU validation.

raw data:

• master: CPU shards 3.272s + 3.670s; GPU shards 0.624s + 0.568s
• this PR: CPU shards 3.336s + 3.260s; GPU shards 0.593s + 0.593s

Testing

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

Low risk: prover-side ownership change only. Rollback is reverting the Arc witness-MLE plumbing.

Follow-ups (optional)

None.

Copilot Reviewer Directive (keep this section)

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