Add Compute Witness Rust integration spike

[safal207]

Summary

Adds a Rust integration spike for ProofPath Compute Witness.

This PR does not implement Rust code yet. It documents the narrow adapter path from the Compute Witness JSON fixture contract into the existing proofpath-verifier crate.

Added

• docs/compute-witness-rust-integration-spike.md

Updated

• docs/compute-witness-pilot-guide.md
  • links the adoption ladder to the Rust integration spike.

Why

The current Compute Witness layer is strong for reviewability:

Python conformance validator
-> fixtures
-> CI
-> audit packet
-> challenge fixtures

The next credibility layer is a production-facing Rust path:

ComputeWitnessJobManifest JSON
-> RequestContext projection
-> proofpath_verifier::verify()
-> ComputeWitnessReceiptDraft
-> later audit/conformance validation

Key design finding

The existing Rust verifier already has:

• RequestContext
• Decision
• ReasonCode
• VerificationResult
• verify(ctx: &RequestContext)
• a proofpath-verify CLI

So Compute Witness should first be integrated as an adapter, not as a rewrite of the core verifier.

Main open question

The current action verifier treats missing causal parent as REJECT, while the current Compute Witness blocked fixture uses BLOCK for MISSING_CAUSAL_PARENT.

Recommended resolution:

Keep generic verifier semantics unchanged.
Add a Compute Witness policy adapter for compute-domain receipt outcomes.

Validation

Documentation-only change.

Recommended checks:

cargo fmt --all -- --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace
python3 scripts/validate_compute_witness.py
python3 scripts/validate_compute_witness.py conformance/compute-witness/challenge_manifest.json

Issue

Refs #62