[Tools] Provide CLI utility in zeroj-tools for converting .zkey files to sparse mmap store format

[satran004]

Background & Objective

ZeroJ supports loading proving keys via memory-mapped sparse stores (Groth16PkStore, ADR-0029/ADR-0035/ADR-0036). Converting a standard snarkjs .zkey into ZeroJ’s sparse .bin store format allows instant memory-mapped startup for provers without re-parsing large .zkey files on every run.

Scope & Tasks

• Add a CLI command in zeroj-tools (e.g. zeroj-tools convert-zkey --in <circuit.zkey> --out <dir> --sparse).
• Support dense and sparse point encoding outputs.
• Output verification key metadata and circuit fingerprint for instant pipeline validation.
• Document usage in docs/groth16-dev-guide.md.

Risk Classification

• R1: Developer tooling & workflow ergonomics.
• Reference: docs/zeroj-comprehensive-review-and-readiness-gemini.md (Section 5.2).

[satran004]

Mostly already done — suggest rescoping to just the CLI wrapper.

The substantive converter exists: ZkeyPkStoreImporter (zeroj-crypto/.../groth16/ZkeyPkStoreImporter.java), delivered under ADR-0031 M2. From its class doc:

Streaming importer for large snarkjs Groth16 .zkey files: converts a ceremony proving key directly into a Groth16PkStore directory without materializing point object arrays — the path for MPC ceremony keys at real scale (a 19M-constraint key is ~30 GB, far past ZkeyImporterBLS381's strict in-memory limits).

It already raw-copies the four G1 sections (the .zkey Montgomery layout is byte-identical to the store's flat-limb layout on little-endian), converts and validates G2, writes VK parts into aux.bin/manifest, and validates in parallel. Groth16PkStore.load(dir) then memory-maps it — which is the "instant memory-mapped startup" this issue asks for.

What is genuinely missing is only the CLI surface. Note zeroj-tools currently has no CLI at all — three library classes (ZkeyContributor, SnarkjsHashToG2, ChaChaRng), no picocli. The only CLI in the repo is CeremonyCli in zeroj-ceremony.

So this is blocked on / must be sequenced with #36, which proposed moving CeremonyCli into zeroj-tools. Both issues add a CLI to the same module; pick one entry point.

Suggested rescope: "Expose the existing ZkeyPkStoreImporter as a zeroj-tools CLI command" — R0/R1, small. Also worth checking task 2 ("dense and sparse point encoding outputs") against what Groth16PkStore already writes before estimating.