feat(zkvm): wire receipt into ProofBundle + witnex-zkvm CLI

[MoonZagallo]

Summary

Wire the Risc0 receipt into the bundle and add a real prover/verifier CLI.

• zkvm/host:
  • attach_proof(bundle) — proves the bundle's trace and embeds the bincode-serialized Receipt into proof.bytes, with the journal as commitment.
  • verify_bundle(bundle) — deserializes the receipt, verifies it against the guest image id, and binds it: journal == bundle.commitment == trace.commitment().
• New witnex-zkvm binary: prove <bundle.json> / verify <bundle.json> — pairs with witnex demo summarize --out b.json to produce a real-proof bundle and check it.

Validated on a real Linux box (dev mode)

$ cd zkvm && RISC0_DEV_MODE=1 cargo test --workspace
test tests::prove_then_verify_recovers_commitment ... ok
test tests::bundle_roundtrips_through_json_and_verifies ... ok
test tests::tampered_trace_fails_bundle_verification ... ok
test result: ok. 3 passed; 0 failed

Receipt now round-trips through ProofBundle JSON and verifies; a forged trace is rejected. The release build (real-proof profile) also compiles clean (~4m on a 1-core box); a full non-dev STARK run is heavy and left to run on demand.

Scope

The main witnex CLI stays structural (no risc0 in the root workspace); real receipt verification is via witnex-zkvm, built with the rzup toolchain on Linux/macOS/WSL. Dev mode skips STARK generation (warnings are expected).