test(rescue): start the drill's kill clock only once the stand-in is armed

BudgetKillDrillTests drove the stand-in deploy with `timeout -k GRACE 3`.
Under load (gate pass 76 ran unit, integration and lint in parallel)
timeout's TERM arrived while bash was still starting, before the
stand-in's `trap cleanup EXIT` existed, so bash died on TERM without the
rescue: test_without_the_window_the_kill_lands_before_the_copy failed
with "124 != 137" and only "kill_epoch=…" on stdout. A 4 s stall before
the trap reproduces it on the old driver; nothing in the merge touched
the rescue code.

The stand-in now writes to an `armed` FIFO right after it installs the
trap. The driver runs `timeout -k GRACE 0 …` (duration 0 disables
timeout's own clock, its TERM-then-KILL handling stays), waits for
`armed`, sleeps until the deadline epoch and sends timeout the same ALRM
its own expiry raises, so TERM still lands at the deadline and KILL
GRACE seconds later, with the same 124/137 exit codes. `wait -n -p`
lets the failure-before-the-deadline drill return as soon as its deploy
ends. No timeout was raised and nothing retries; with the 4 s stall all
three drills pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/python/scripts/tests/deploy/utils/rescue/test_window.py

@@ -44,7 +44,22 @@ if [ "${EPOCHS}" = true ]; then
 	export INFINITO_CI_DISTRO_KILL_EPOCH="$((deadline + GRACE))"
 fi
 echo "kill_epoch=$((deadline + GRACE))"
-timeout -k "${GRACE}" "$((deadline - now))s" bash "${SANDBOX}/stand-in.sh" "$@"
+mkfifo "${SANDBOX}/armed"
+timeout -k "${GRACE}" 0 bash "${SANDBOX}/stand-in.sh" "$@" &
+deploy="$!"
+read -r _ <"${SANDBOX}/armed"
+left="$((deadline - $(date +%s)))"
+sleep "$((left > 0 ? left : 0))" &
+clock="$!"
+status=0
+wait -n -p first "${deploy}" "${clock}" || status="$?"
+if [ "${first}" = "${clock}" ]; then
+	kill -ALRM "${deploy}"
+	wait "${deploy}" || status="$?"
+else
+	kill "${clock}"
+fi
+exit "${status}"
 """
 
 STAND_IN = """\
@@ -65,6 +80,7 @@ cleanup() {
 	date +%s >"${SANDBOX}/rescued"
 }
 trap cleanup EXIT
+echo armed >"${SANDBOX}/armed"
 "$@"
 """