refactor(runtime): decouple supervisor access and boundary audit validation - #4106

Summary

Extract the backend-neutral runtime foundations from #4084 so they can be reviewed and merged independently of MXC. Keep the existing IsolationBackend and authenticated Sandbox Protocol; remove Unix assumptions from supervisor access and allow implementing backends to validate their own opaque audit evidence.

Related Issue

Follow-up to #1737 (RFC 0012). Prerequisite for #4084.

Changes

• Keep gateway authentication, canonical process attachment, and forwarding independent of optional Unix SSH access.
• Make TCP readiness portable while preserving initial acceptance, disconnect/reconnect gating, and teardown.
• Retain local Unix signal support, including SIGQUIT, through a separate adapter to backend-neutral signals.
• Inject a BoundaryAuditValidator into the shared runtime; strict Linux evidence validation remains the default, and asserted properties must match validated evidence.
• Include supervisor libraries in native Windows checks/tests without adding a Windows isolation backend or executable.
• Document the control-plane and evidence-validation seams and update Windows contributor guidance.
• Correct one pre-existing Windows unit-fixture constructor call so the base's all-target validation can run independently.

No UI policy, explicit-proxy networking, MXC implementation, or public protobuf changes are included.

Testing

• Native x64 workspace check: mise run --skip-tools windows:check:x64
• 250 focused native tests passed across openshell-supervisor, openshell-supervisor-process, and openshell-sandbox-backend.
• mise run pre-commit passes through the commit hook.
• Unit tests added/updated; existing confirmation mismatch and invalid-evidence tests preserved.
• Linux runtime and sandbox E2E regression validation; requires Linux CI/host.
• Native ARM64 validation.

Windows control-plane tests are not MXC containment qualification. This PR remains draft pending cross-platform runtime validation.

Checklist

• Follows Conventional Commits.
• Commits are signed off (DCO).
• Crate implementation documentation and related contributor skill guidance updated.

Stack

1. This PR: main -> codex/runtime-foundations.
2. feat(mxc): integrate Windows runtime on portable isolation foundations #4084: codex/runtime-foundations -> drew-mxc.

Merge this PR first. Retarget #4084 to main afterward; if the base is squash-merged, merge updated main into the child branch so its comparison stays incremental.