Hand lid_policy back unset where the machine had it unset

motion.lid-policy-hold read the setting as

    was = (fc.settings() or {}).get("lid_policy") or "cancel"

and wrote `was` back at the end. On a machine that has never set the
policy the setting reads as the empty string and behaves as cancel, so
the `or` turned "unset" into the word and the test handed the machine
back carrying a setting it did not arrive with. The hand-back reported
it, restored it, and failed the test.

The value is now captured exactly, empty included, and restored as
captured; the default belongs to reading the value, never to writing it
back. The read-back check gets the same default, so it no longer compares
None with the empty string. lid_policy_in_force carries the effective
policy into the evidence, which is what the old expression was reaching
for.

Found on the bench reference, on the run after the position dead band let
motion.lid-cancel-home through. The four other places in the suite that
read a setting with `or` are safe: two default to the empty string, which
is what unset is, and two feed a check rather than a restore. The shared
Restore helper captures raw values and writes the empty string back for
unset, as this now does.

Only this test's earlier passes are invalidated: the change is inside its
own body, and the per-test source hash of the other eleven motion tests
is unchanged (checked against the file before the edit).

forgetest/forgetest/suite/motion.py

@@ -1285,8 +1285,13 @@ def interlock_cancel_home(ctx):
 def lid_policy_hold(ctx):
     ev = ctx.evidence
     fc = ctx.forgectrl
-    was = (fc.settings() or {}).get("lid_policy") or "cancel"
+    # What the machine had, exactly: an unset lid_policy reads as the empty
+    # string and behaves as cancel, and writing the word back where the
+    # machine had nothing is a leftover the hand-back reports. The default
+    # is for reading the value, never for restoring it.
+    was = (fc.settings() or {}).get("lid_policy", "")
     ev["lid_policy_before"] = was
+    ev["lid_policy_in_force"] = was or "cancel"
     st, _b = fc.post("/settings", data={"lid_policy": "hold"})
     ctx.check(st == 200, "could not set lid_policy=hold (%s)", st)
     ctx.check(((fc.settings() or {}).get("lid_policy")) == "hold", "lid_policy did not take")
@@ -1333,7 +1338,7 @@ def lid_policy_hold(ctx):
         machine_idle(ctx)
     finally:
         st, _b = fc.post("/settings", data={"lid_policy": was})
-        ev["lid_policy_restored"] = (fc.settings() or {}).get("lid_policy")
+        ev["lid_policy_restored"] = (fc.settings() or {}).get("lid_policy", "")
         ctx.log("lid_policy restored to %s", ev["lid_policy_restored"])
     ctx.check(ev["lid_policy_restored"] == was, "lid_policy was not restored to %r", was)
     ctx.log("PASS: lid_policy=hold parked the job in Door and the cycle start finished it (%.3f mm)",