fix(scripts): reset-aware handshake refuses a null reset value; document additive timeouts (#36 review)

A null resetValue encoded to null, which wrote an empty string and then handed the
waiter a null target (= any change), fast-pathing on the stale cache and reinstating
the race the overload exists to close. The context parameter is now non-nullable and
the accessor throws ArgumentException before any write. Also: additive-timeout note on
all surfaces, batch-failure message no longer names the bool overload, and the
error-message/batch-failure branches are pinned.

Claude-Session: https://claude.ai/code/session_01XPsuy2ZZ5Gbyfpt8PrK5BV

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScopeAccessorTests.cs

@@ -341,4 +341,33 @@ public class AttributeAccessorWaitAsyncTests : TestKit, IDisposable
         // Nothing else was written: no batch, no trigger.
         probe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
     }
+
+    /// <summary>
+    /// Gitea #36 review: a null <c>resetValue</c> would encode to null, which writes an empty
+    /// string on leg 1 and then hands the waiter a null target — "any change" — which fast-paths
+    /// on the stale cached value and reinstates exactly the race this overload exists to close.
+    /// The accessor rejects it before anything reaches the InstanceActor.
+    /// </summary>
+    [Fact]
+    public void WriteBatchAndWaitAsync_ResetAware_NullResetValue_ThrowsBeforeAnyWrite()
+    {
+        var probe = CreateTestProbe();
+        var ctx = MakeContext(probe.Ref);
+
+        var acc = new AttributeAccessor(ctx, "RightMESReceiver");
+
+        // Block body (not an expression lambda) so this binds the Action overload: the throw is
+        // SYNCHRONOUS — a faulted task would leave nothing for Assert.Throws to catch.
+        var ex = Assert.Throws<ArgumentException>(() =>
+        {
+            _ = acc.WriteBatchAndWaitAsync(
+                new Dictionary<string, object?> { ["MoveOutBatchID"] = 28950 },
+                "MoveOutFlag", true, "MoveOutCompleteFlag", true, TimeSpan.FromSeconds(25),
+                "MoveOutCompleteFlag", null);
+        });
+        Assert.Equal("resetValue", ex.ParamName);
+
+        // Thrown before leg 1: the reset write never left the accessor.
+        probe.ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+    }
 }