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/Actors/HandshakeResetEchoTests.cs

@@ -109,7 +109,7 @@ public class HandshakeResetEchoTests : TestKit, IDisposable
         var call = ctx.WriteBatchResetAndWait(
             Values(28950), "MoveOutFlag", AttributeValueCodec.Encode(true),
             "MoveOutCompleteFlag", AttributeValueCodec.Encode(true), TimeSpan.FromSeconds(10),
-            "MoveOutCompleteFlag", AttributeValueCodec.Encode(false), TimeSpan.FromSeconds(10));
+            "MoveOutCompleteFlag", AttributeValueCodec.Encode(false)!, TimeSpan.FromSeconds(10));
 
         // Leg 1 reaches the device as a plain tag write; the DCL acks it but the device has
         // not echoed anything yet.
@@ -152,7 +152,7 @@ public class HandshakeResetEchoTests : TestKit, IDisposable
         var call = ctx.WriteBatchResetAndWait(
             Values(28950), "MoveOutFlag", AttributeValueCodec.Encode(true),
             "MoveOutCompleteFlag", AttributeValueCodec.Encode(true), TimeSpan.FromSeconds(10),
-            "MoveOutCompleteFlag", AttributeValueCodec.Encode(false), TimeSpan.FromMilliseconds(700));
+            "MoveOutCompleteFlag", AttributeValueCodec.Encode(false)!, TimeSpan.FromMilliseconds(700));
 
         var reset = dcl.ExpectMsg<WriteTagRequest>(TimeSpan.FromSeconds(5));
         dcl.Reply(new WriteTagResponse(reset.CorrelationId, true, null, DateTimeOffset.UtcNow));