[elezar]

Summary

Move the Rust types for the documents a compute driver hands to the supervisor and the sandbox boundary into openshell-driver-sdk, so the Docker, Podman, and VM drivers no longer depend on openshell-core, openshell-sandbox-backend, or openshell-isolation-interface. This stacks on #5405, which carves out the SDK and deliberately left these types in place.

Related Issue

Part of #5456. This PR covers one part of that issue: removing the driver crates' dependency on the core crates. It does not specify the documents in a language-neutral form, and it does not cover the shared-supervision registration; those remain open in #5456.

Stacked on #5405: the base branch is refactor/5383-carve-out-driver-sdk/elezar, and this PR should merge after it.

Changes

• Add a legacy_contract module to openshell-driver-sdk behind a legacy-contract feature. It holds BoundaryConfig, SandboxRuntimeDescriptor, SandboxTransport, BoundaryListener, the TLS types, GatewayVerificationKey, ResolvedWorkloadIdentity, ProvisioningEvidence, Sha256Digest, BackendError, BackendDescriptor, generate_sandbox_tls_material, the route handle check, and the backend name, resource claim, and supervisor CA path constants.
• Re-export every moved item from openshell-sandbox-backend and openshell-isolation-interface at its current path, so each item has one definition and existing callers are unchanged. The From impl from the registered endpoint transport became transport_from_registration, because the orphan rule forbids it in either crate.
• Repoint the Docker, Podman, and VM drivers to the SDK and drop their dependencies on the three core crates. The VM driver also drops openshell-policy; it used only the sandbox UID range, which the SDK already defines.
• Document the module as legacy in the SDK README and in the module docs: it describes the format that exists today, accepts no new fields, and is removed once a replacement ships. The README also records the replacement options and a possible migration path. Nothing about the replacement is decided here.
• Add golden JSON examples and a round-trip test that pins the encoding of each document and checks that readers reject invalid ones.

Testing

• Checks appropriate to the affected code and behavior pass
  • cargo check --workspace --all-targets and cargo clippy on the touched crates are clean, and the pre-commit hooks pass.
  • cargo tree shows the three drivers no longer depend on openshell-core, openshell-sandbox-backend, openshell-isolation-interface, or openshell-policy for normal builds.
• Unit tests added/updated (if applicable)
  • openshell-driver-sdk, openshell-isolation-interface, openshell-sandbox-backend, and the Docker and Podman drivers pass. Six VM driver tests fail on a machine without mke2fs (e2fsprogs); they need that tool and I did not run them on the base branch.
• E2E tests added/updated (if applicable)
  • The Docker, Podman, and VM e2e suites have not been run, so "no behavior change" is verified only by compilation and unit tests.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)