Preserve prospective-study observations from short-lived processes #74

The optional prospective-study recorder can lose queued lifecycle observations when a one-shot owner process exits. Its consumer thread is a daemon, so enqueueing an observation does not prove it survives interpreter shutdown.

Reported in PR #73 review, and reproduced during review of the current-attention upgrade. The defect remains at 3a198291a29cbb055f819e2a7afdfb10142ce8b4; that upgrade does not enable the study.

Before activating the study or making completeness claims:

• Provide a durable detached handoff or bounded shutdown flush for short-lived callers.
• Exercise a real child process that emits opened/chosen/settled observations and exits immediately, then verify the persisted observations from another process.
• Preserve nonblocking foreground behavior under a busy recorder and make any bounded loss explicit.

Keep this repair separate from canonical attention, ownership and semantic settlement. The study remains default-OFF until this and the cohort-feasibility defect are closed.