Disk-spill for proving 32M program

[gabrielbosio]

Enables proving programs that exceed available RAM by spilling intermediate data (traces, LDE evaluations, Merkle trees) to disk via mmap.

Extends #444 with reduced time regression.

On a 128 GB machine with 4 GB of swap, the VM can prove fib_iterative_16M on main and cannot prove fib_iterative_32M because of OOM error. This PR allows the VM to prove fib_iterative_32M.

Gated behind disk-spill cargo feature.

Enables logging by making existing log::* function calls to actually log the expected messages and adds logging to indicate storage mode used when disk spill is enabled.

Bench Results

Program: fib_iterative_32M.elf
Command executed 3 times:

./target/release/cli prove executor/program_artifacts/asm/fib_iterative_32M.elf -o /tmp/proof.bin --time

Proving time: ~173s

[gabrielbosio]

/bench

[github-actions]

Benchmark — fib_iterative_8M (median of 3)

_{Table parallelism: auto (cores / 3)}

Metric	main	PR	Δ
Peak heap	53104 MB	52611 MB	-493 MB (-0.9%) ⚪
Prove time	25.197s	25.376s	+0.179s (+0.7%) ⚪

✅ No significant change.

✅ Low variance (time: 0.9%, heap: 1.0%)

_{Commit: c5a41dc · Baseline: built from main · Runner: self-hosted bench}

[gabrielbosio]

/bench

[gabrielbosio]

/bench

[gabrielbosio]

/bench

[gabrielbosio]

/bench

[gabrielbosio]

/bench

[gabrielbosio]

/bench

[gabrielbosio]

/codex /claude