Fix cross-session orphan Rescue settlement

[vitry]

Summary

• Safely settle provable orphaned writable Rescue jobs during reservation and SessionEnd without transferring ownership.
• Add existing-broker-only protocol capability so SessionEnd cannot lazily spawn ZCode.
• Fence completion/artifact races, preserve guards on unacknowledged stops, and document the lifecycle contract.

Test plan

• npm run check
• Focused orphan/session-end/client/release-contract suites
• Standards and Spec reviews from c753155
• Real qualified E2E remains explicitly opt-in and skipped in this environment