[draft] Unmount: separate the request deadline from the join observation deadline #4

Draft — review before filing upstream. Design cleanup; no behavior change on its own.

Current state

DRAIN_WAIT (5 s) bounds both the request wait and the observation of the receive-loop join. Ephemeral-AI-Lab#497 adds JOIN_WAIT = 1 s for the join once requests are quiescent; the request wait still uses the full DRAIN_WAIT.

Proposal

Make the two waits explicit in code and docs (two names/constants, or a small struct), so a future fix cannot "just raise DRAIN_WAIT" again. Defaults unchanged: 5 s requests, 1 s join after quiescence.

Data behind the defaults

• normal unmount: n=20, 17–44 ms with the grace; 20–82 ms without.
• 500 ms was rejected because one healthy unmount took 374 ms under load; 1 s keeps ~2.7× margin.
• early-mount stall: 5.0–5.6 s with the 5 s wait; 1.02–1.47 s with 1 s.

Acceptance

• names/doc explain which wait covers what; no behavioral change; existing tests unchanged.
• the abort still fires on the request deadline (DRAIN_WAIT); the join grace only bounds observation — a split must not change that.