feat(mtconnect): serve DATA_SET/TABLE as compact JSON strings; park EVENT enumerations (#650)

Parser captures Entry/Cell content; the observation index merges /sample deltas
into the /current baseline (resetTriggered replaces, removed deletes, UNAVAILABLE
drops). Fixture gains a TABLE item and periodic DATA_SET/TABLE churn.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectObservationIndexTests.cs

@@ -589,11 +589,12 @@ public sealed class MTConnectObservationIndexTests
 
     /// <summary>
     ///     End-to-end through the real parser: the entry list that concatenates to the single token
-    ///     "12" is caught by the parser's structured flag and never surfaces as a Good value. This is
-    ///     the leg that would regress silently if the parser stopped setting the flag.
+    ///     "12" must never surface as that noise. Since #650 it is served as JSON carrying the keys
+    ///     (full semantics in <c>MTConnectDataSetTests</c>); the structured-without-entries shape above
+    ///     stays BadNotSupported.
     /// </summary>
     [Fact]
-    public void A_parsed_data_set_observation_never_surfaces_as_a_good_value()
+    public void A_parsed_data_set_observation_never_surfaces_as_concatenated_text()
     {
         const string xml = """
             <MTConnectStreams>
@@ -612,8 +613,8 @@ public sealed class MTConnectObservationIndexTests
 
         var snap = idx.Get("ds1");
 
-        snap.StatusCode.ShouldBe(BadNotSupported);
-        snap.Value.ShouldBeNull();
+        snap.StatusCode.ShouldBe(Good);
+        snap.Value.ShouldBe("""{"V1":1,"V2":2}""");
     }
 
     /// <summary>An ordinary text observation is not structured — the flag must not be over-applied.</summary>