forgetest: the latch-unlock gate waits for the safety chain to release

The gate that refuses a latch unlock while the chain may hold HV_ENABLE
up (charge_pump_alive or a pulse engine not idle) ran at the start of
phases B, U and K3 of kernel.fire-line, within a second of the previous
phase's run. A run feeds the charge-pump watchdog every 200 ms and the
one-shot holds ALIVE for 0.45 s after the last feed, so the gate read
alive=1 and refused: the first bench run of the gate (forgefirm
64f552fc; the laser_pgood gate before it was vacuous) failed phase B on
image 20260902144848.

wait_hv_off() polls the chain for up to 3 s before it refuses, logs the
release when it was not immediate and records every wait in the
evidence (hv_release_s). require_hv_off and check_hv_off use it. The
bench scripts that copy the gate (fire_test.py per phase,
gate_a_kernel_drills.py K3 after K2) get the same wait.

Bench: kernel.fire-line PASS on 20260902144848 with the chain released
after 0.41 s at each of the three phase boundaries. Host:
tests/test_kernel_suite.py covers release inside the window, a chain
held past it, and a chain already off.

Catalog consequence: the kernel.* implementation hashes move (the suite
file changed); the kernel set re-ran and passed.

scripts/bench/gate_a_kernel_drills.py

@@ -65,6 +65,18 @@ def hv_off_reason():
         return 'charge_pump_alive=%s state=%s' % (alive, state)
     return None
 
+
+def wait_hv_off(timeout_s=3.0):
+    # A run feeds the charge-pump watchdog every 200 ms and the one-shot
+    # holds ALIVE for 0.45 s after the last feed, so a phase that follows a
+    # run finds the chain still up for under a second: wait for the release.
+    t0 = time.time()
+    why = hv_off_reason()
+    while why is not None and time.time() - t0 < timeout_s:
+        time.sleep(0.05)
+        why = hv_off_reason()
+    return why
+
 def rd_pos():
     with open('/sys/glowforge/cnc/position', 'rb') as f:
         raw = f.read(32)
@@ -220,7 +232,7 @@ def drill_k2():
 
 
 def drill_k3():
-    why = hv_off_reason()
+    why = wait_hv_off()
     if why is not None:
         print('ABORT: the safety chain is not holding HV off (%s) - refusing latch unlock' % why)
         return 1