Separate runtime eligibility from traffic allocation

A weighted rollout can select a runtime mode only on instances that have passed local startup checks for that mode. Keep an explicit eligibility setting; when it is off, preserve the old runtime regardless of remote weights. On eligible instances, fetch and validate weights in the background, retain the last valid policy on read failure, and select once per session. Missing policy should use the existing eligible behavior. Keep cleanup and error handling on the selected path, and record a privacy-safe session correlation plus policy revision so observed traffic can be attributed. Remote rollback changes new sessions after refresh; existing sessions remain pinned.

Applies when: A service adds a per-session runtime behind a central weighted rollout. Keywords: weighted rollout eligibility runtime session cache fallback revision rollback