orb(attest): reproducible pinned replay-runner image + digest manifest enforced in CI

[JSONbored]

Context

Part of the attested-evaluation epic (#8534). Attestation over an unpinned, irreproducible workload image is theater: the measurement must correspond to an image anyone can rebuild and check. The replay runner needs a minimal, deterministic image whose digest is recorded where the harness and verifier can consume it.

Requirements

1. A minimal container image for the backtest replay runner: pinned base image by digest, lockfile-only dependency install, no network access required at runtime, no secrets baked in, nothing beyond what replay needs.
2. CI builds the image and records its digest into a committed manifest; the attested-run harness reads expected-measurement input from that manifest. Digest drift (manifest vs fresh build) fails CI.
3. Rebuild reproducibility documented: the exact command sequence a third party runs to reproduce the digest, and the known caveats if full bit-reproducibility isn't achievable (state precisely what is and isn't reproducible).

Deliverables

• Dockerfile + CI job + committed digest manifest + drift test + the reproducibility doc section.

Expected outcome

The measurement inside an attestation envelope corresponds to an image whose provenance anyone can independently rebuild and confirm.

[linear-code]

JSO-6388