fix(historian): ContinuousHistorization CommitMode code default is PerEntry, matching appsettings/docker-dev/fallback (lmxopcua#776)

The Lane H2 remediation made Periodic(100 ms) the options-class default, but
appsettings.json, docker-dev and Program.cs's bad-value fallback all used
PerEntry, so effective behaviour never changed and the docs contradicted the
shipped config. The options default is now PerEntry; Periodic stays opt-in
(<= CommitIntervalMs of crash loss in exchange for throughput). Program.cs now
uses ContinuousHistorizationOptions.ResolveCommitMode() so the fallback shares
the single DefaultCommitMode constant. docs/Historian.md updated.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ContinuousHistorizationOptionsTests.cs

@@ -1,4 +1,5 @@
 using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Runtime.Historian;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.Historian;
@@ -36,6 +37,16 @@ public sealed class ContinuousHistorizationOptionsTests
             }.Validate(),
             m => m.Contains("CommitIntervalMs"));
 
+    [Fact]
+    public void Unparseable_commit_mode_falls_back_to_the_options_default()
+    {
+        var defaults = new ContinuousHistorizationOptions();
+        Assert.Equal(HistorizationCommitMode.PerEntry, defaults.ResolveCommitMode());
+        Assert.Equal(ContinuousHistorizationOptions.DefaultCommitMode, new ContinuousHistorizationOptions { CommitMode = "garbage" }.ResolveCommitMode());
+        Assert.Equal(Enum.Parse<HistorizationCommitMode>(defaults.CommitMode), new ContinuousHistorizationOptions { CommitMode = "garbage" }.ResolveCommitMode());
+        Assert.Equal(HistorizationCommitMode.Periodic, new ContinuousHistorizationOptions { CommitMode = "periodic" }.ResolveCommitMode());
+    }
+
     /// <summary>
     ///     Pins the shipped defaults, which are a tuned set rather than arbitrary numbers: the drain is
     ///     bounded-parallel over an in-memory peek (so a wide <c>DrainBatchSize</c> is cheap and a narrow
@@ -49,7 +60,7 @@ public sealed class ContinuousHistorizationOptionsTests
     {
         var options = new ContinuousHistorizationOptions();
 
-        Assert.Equal("Periodic", options.CommitMode);   // 100 ms group commits — PerEntry capped ingest at 1 / fsync latency
+        Assert.Equal("PerEntry", options.CommitMode);   // matches appsettings.json, docker-dev and the bad-value fallback (lmxopcua#776)
         Assert.Equal(100, options.CommitIntervalMs);
         Assert.Equal(1024, options.DrainBatchSize);
         Assert.Equal(8, options.DrainConcurrency);