feat(isolation): add RFC 0012 backend contract

[drew]

Summary

Define RFC 0012's runtime-selectable Isolation Backend contract and its security invariants. This is PR 1 of 4 in the review stack and introduces no runtime activation.

Stack: this PR → #2942 → #2945 → #2965.

The contract has one boundary entry operation: attach. A trusted TopologyDescriptor may identify an existing resource or carry prepared inputs that a backend uses to establish the boundary during attach; the compute driver retains durable resource deletion and reconciliation ownership.

Review-only prototype: The original code author is @jganoff, and the original implementation source is jganoff/OpenShell:jganoff/rfc-0012-impl. This reconstructed stack is for reviewing and prototyping RFC 0012; we do not intend to merge these PRs. The stack is rebased onto current main and integrated with the current canonical process and networking architecture.

Related Issue

Part of #1737. The final PR in this stack carries the Docker proof.

Changes

• Add the RFC 0012 proposal, codebase grounding, and topology matrix.
• Add the single openshell-isolation-interface crate with consuming Bound → Ready → Running lifecycle states.
• Add exact descriptor/backend/version resolution with no fallback.
• Add placement-neutral process, exec, terminal, port-forward, DNS, and network-mediation contracts.
• Expose a backend-provided trusted host-gateway dial target without changing policy-visible hostnames.
• Add contract conformance and fail-closed behavior tests.
• Keep backend implementations out of the interface crate.

Testing

• cargo test -p openshell-isolation-interface — 21 passed on the complete stack
• Integrated stack compiles across sandbox, supervisor, and affected drivers
• mise run pre-commit passes on the complete stack

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture and agent crate inventories updated

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