forgetest: the button's commanded level, and one app cancel instead of two

laser.arm-wait-lid failed its first bench run on the witness, not the
machine: the cancel relocked, disarmed and emitted nothing, but the
"button dark" check read the LEDs' brightness the instant after, and
the smooth trigger fades it. The controller writes `target`, so that is
what hw.button_leds() reads now (brightness where no target exists),
and check_button_dark waits a few seconds for the command to land.

The operator's campaign showed cloud.oversize-stream and
cloud.pause-cancel-paths both cancelling a print from the app. A print
longer than the ring has to be ended that way, and the operator is at
the app for it anyway, so the app cancel lives there and is judged in
full (the abort tail: stop, park to the job start, relock, disarm, the
button dark, ':cancelled'); cloud.pause-cancel-paths loses its second
print and becomes cloud.paused-lid-cancel - one print, one arm press.
Replays follow. Catalog consequence: the two cloud tests and the laser
module are re-required; the count stays 43.

forgetest/forgetest/hw.py

@@ -264,14 +264,20 @@ def leds_root():
 
 
 def button_leds():
-    """The three button LED brightnesses, or None where unreadable."""
+    """The three button LEDs' commanded levels (the smooth trigger's
+    `target`, which the controller writes; `brightness` follows it with
+    a fade and is read where no target exists), or None where unreadable."""
     out = []
     for name in BUTTON_LEDS:
-        try:
-            with open(leds_root() + name + "/brightness") as f:
-                out.append(int(f.read().strip()))
-        except (OSError, ValueError):
-            out.append(None)
+        val = None
+        for attr in ("target", "brightness"):
+            try:
+                with open(leds_root() + name + "/" + attr) as f:
+                    val = int(f.read().strip())
+                break
+            except (OSError, ValueError):
+                continue
+        out.append(val)
     return out