Found by running the round-8 fix in the real container instead of trusting the
unit test: a directory created 2ms AFTER `_PROCESS_START` was stamped with an
mtime 2ms BEFORE it, so the sweep deleted it as a leftover. The two sides come
from different clocks — a file's mtime from the kernel's coarse clock (one timer
tick of lag), `time.time()` from the fine one — and the unit test could not see it
because it supplies an artificial cutoff.
The window is small and lands exactly where it hurts: the sweep runs at boot, so
the session at risk is one started in the first instants after startup — the case
the guard exists for. Keep-side slack of 2s; the cost is that a leftover from the
last seconds before a restart survives until the next one.
Verified in the container: a backdated leftover is swept, a directory created by
this run is kept, and a registered live session with an old mtime is kept — 1 of 3.
Test pins the skew (mutation-checked: with the grace at 0 it goes red).
peter
5fe6d32d38