feat(oci): add S12 Step 5 helpers (digest, mediatype, errors, name)

Hayley Swimelarrequested to merge
hswimelar/oci-local-step-5 into main

Why

S12 Step 5 (Container/OCI local) helpers: digest parsing/canonicalization, manifest mediatype detection, the OCI error envelope, and image-name path validation. Step 6's dispatcher and every endpoint MR after it consume these.

Plan: docs/plans/2026-05-15-oci-local.md. Spec: docs/specs/S12-container-oci-local.md.

Context for LLM agents

Rationale

  • NewInternalEnvelope(_ error) accepts an err parameter it intentionally drops. Alternative considered: a logger-coupled helper that logs and returns the envelope. Rejected because it would force the format package to import the logger; the no-internal-detail invariant is enforced by call-site discipline plus TestInternalEnvelopeContainsNoInternalDetails.

Non-goals

  • Dispatcher and route wiring (Step 6).
  • GC coordination and reachability events (deferred to S20).
  • Storage accounting writes (deferred to S22).
  • A typed Detail shape that prevents callers from passing internal data into NewErrorEnvelope (Step 6 follow-up).

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