A marker proved a turn had started, never that its writer had died, so a
second backend resuming the same session over a shared HERMES_HOME read a
sibling's live marker as crash evidence and started a duplicate turn over it.
record_turn_start now stamps writer_pid and, when readable, writer_start_time
(process create time, so a recycled pid cannot pass as the original writer)
and logs the identity — never the prompt. New marker_writer_state(entry)
answers alive/dead/unknown from active_sessions._pid_liveness, and
_maybe_schedule_auto_continue holds back when the writer is alive and is not
this process: no continuation, no "Resuming interrupted turn…" frame, and the
marker stays for its owner to clear.
Adds the two-backend regression: a real second process writes the marker and
keeps running while B resumes S (nothing scheduled, marker intact), then exits
and the same resume does schedule — so the live-writer gate is what defers B,
not some other switch.
30e025c0c4