deploy-gate flake: test_frontdoor TestSourceSideSequencing.test_stalled_member_drop_does_not_wedge_the_group fails under hub load #1997

Failed in 2 of 4 deploy-gate.sh --scope engine runs on erlich on 2026-10-28 (load 16–19 from Julia solves). It passed 1/1 when run alone, both on lane/0.3.11 and on the #1995 branch, and neither touches ops/hub/hub-frontdoor.py. Most likely a wall-clock stall threshold in the test that a loaded box overshoots.

Fix: derive the stall/drop timing from an injected clock or scale it by a measured baseline. Keep the test's claim (the drop path never holds the group lock while closing) intact. Do not loosen it into a skip.

AC: the test passes 50/50 with stress-ng --cpu 24 running alongside, and still fails when the lock is held during close (positive control).