fix(genrm): bound incomplete cohorts and preserve cohort accounting #3181

Priority

Wave 0 — urgent GenRM correctness. The bounded-wait fix does not depend on the final #2135 outcome model.

Problem

Cohort verification waits for all expected members. One missing member can leave every arrived member blocked indefinitely, while an unmarked partial reward can silently change evaluation denominators and training prompt groups.

Essential scope

• A configured finite cohort-completion deadline.
• Cancellation-safe removal or resolution of every waiter.
• Cohort identity sufficient to isolate concurrent cohorts and fence late output from an older attempt.
• An initial explicit incomplete policy: fail the whole cohort, or return a completed result marked mask_sample=True only when usable output genuinely exists.
• Never emit an ordinary unmarked reward from incomplete membership.
• Focused tests for full completion, timeout, cancellation, concurrent cohorts, and the final-arrival/timeout race.
• Basic bounded logs or counters for timeout and final disposition.

Later integration

• Translate terminal cohort failure into feat(core): define typed rollout outcomes and recovery #2135's durable outcome after that contract stabilizes.
• Add richer expected/arrived/missing accounting and retry orchestration only when a caller defines a policy that consumes them.
• Export telemetry through feat(observability): standardize error-recovery telemetry #3182.

Dependencies

No dependency for the Wave 0 timeout. #2135 and #3179 are later integration dependencies, not prerequisites to stop the hang.

Implementation candidate

#2385 is the starting point. Its partial-cohort reward behavior must fail or mask explicitly.

Acceptance criteria

• Every cohort wait ends through completion, explicit timeout failure, or cancellation.
• Concurrent cohorts cannot share waiters or wake one another.
• A late older completion cannot replace a newer accepted disposition.
• Incomplete membership never produces an unmarked ordinary reward.
• Timeout, cancellation, and final-arrival races leave no blocked futures or retained cohort state.