Implement durable capability transitions and stale-work fencing in the hosted runtime

[dinpd]

Parent: #78
Depends on: task capability-state contract and local enforcement issues under #78

Outcome

Implement tenant-scoped, durable, atomic capability transitions in the Cloudflare runtime and fence parallel work authorized under an obsolete state.

Scope

• Durable task graph records and versioned capability state.
• Compare-and-set or single-writer transitions.
• Transition pending/committed/failed lifecycle.
• Required enforcement acknowledgements and timeout policy.
• Stale authorization and execution-result handling.
• Concurrent requests, retries, cancellation, and idempotent transition replay.
• Tenant-scoped lifecycle and audit endpoints.
• State expiry and cleanup behavior.

Non-goals

• Implementing a network gateway.
• Releasing protected data through browser-facing management APIs.
• Distributed consensus beyond the declared AgentPass deployment boundary.

Acceptance criteria

• Concurrent transitions serialize deterministically.
• Calls authorized under an obsolete version cannot execute after a narrowing transition commits.
• A protected result remains held until all required acknowledgements are recorded.
• Missing acknowledgement, timeout, storage error, or compare-and-set conflict fails closed.
• Identical transition retries return the committed result; changed retries are rejected.
• Audit records reconstruct pending, acknowledged, committed, failed, timed-out, and stale-work outcomes.
• Tenant isolation and authorization tests cover all lifecycle endpoints.
• Failure-injection tests cover concurrency, partial acknowledgement, Durable Object restart, duplicate delivery, and expired tasks.

Security considerations

A stale allow must never survive a committed restriction. Management, acknowledgement, and read APIs must not expose protected results or permit callers to forge another enforcement component's acknowledgement.