fix(commons): resolve Commons-005..007,009..012 — OPC UA parse status, TryConvert correctness, Result null guard, invariant formatting, doc refresh

src/ScadaLink.Commons/Types/DynamicJsonElement.cs

@@ -55,8 +55,19 @@ public class DynamicJsonElement : DynamicObject
 
     public override bool TryConvert(ConvertBinder binder, out object? result)
     {
+        // Conversion to object (or dynamic): never null out a present value. Return the
+        // unwrapped value for scalars, this wrapper for objects/arrays, and null only
+        // when the element is genuinely JSON null.
+        if (binder.Type == typeof(object))
+        {
+            result = _element.ValueKind == JsonValueKind.Null ? null : Wrap(_element);
+            return true;
+        }
+
         result = ConvertTo(binder.Type);
-        return result != null || binder.Type == typeof(object);
+        // A non-object target with a null result means ConvertTo could not handle the
+        // element/type pair — report failure so the binder surfaces a binding error.
+        return result != null;
     }
 
     public override string ToString()

src/ScadaLink.Commons/Types/Result.cs

@@ -14,6 +14,9 @@ public sealed class Result<T>
 
     private Result(string error)
     {
+        // A failed Result must always carry a usable message — Result is the
+        // system-wide error-handling type, and consumers log/display Error directly.
+        ArgumentException.ThrowIfNullOrWhiteSpace(error);
         _value = default;
         _error = error;
         IsSuccess = false;
@@ -33,6 +36,11 @@ public sealed class Result<T>
 
     public static Result<T> Success(T value) => new(value);
 
+    /// <summary>
+    /// Creates a failed result carrying the given error message.
+    /// </summary>
+    /// <exception cref="ArgumentNullException"><paramref name="error"/> is null.</exception>
+    /// <exception cref="ArgumentException"><paramref name="error"/> is empty or whitespace.</exception>
     public static Result<T> Failure(string error) => new(error);
 
     public TResult Match<TResult>(Func<T, TResult> onSuccess, Func<string, TResult> onFailure) =>

src/ScadaLink.Commons/Types/ValueFormatter.cs

@@ -1,4 +1,5 @@
 using System.Collections;
+using System.Globalization;
 
 namespace ScadaLink.Commons.Types;
 
@@ -9,21 +10,36 @@ namespace ScadaLink.Commons.Types;
 public static class ValueFormatter
 {
     /// <summary>
-    /// Formats a value for display as a string. Returns the value's natural
-    /// string representation for scalars, and comma-separated elements for
-    /// array/collection types.
+    /// Formats a value as a string. Returns the value's string representation for
+    /// scalars and comma-separated elements for array/collection types.
     /// </summary>
+    /// <remarks>
+    /// Formatting is <see cref="CultureInfo.InvariantCulture">culture-invariant</see>:
+    /// numbers and <see cref="DateTime"/> values render the same regardless of the
+    /// server/thread locale. This is required because the formatter feeds non-UI
+    /// contexts (gRPC stream events, logs, diff display) where locale-dependent
+    /// output (decimal separators, date order) would be inconsistent.
+    /// </remarks>
     public static string FormatDisplayValue(object? value)
     {
         if (value is null) return "";
         if (value is string s) return s;
-        if (value is IFormattable) return value.ToString() ?? "";
+        if (value is IFormattable formattable)
+            return formattable.ToString(null, CultureInfo.InvariantCulture) ?? "";
 
         if (value is IEnumerable enumerable)
         {
-            return string.Join(",", enumerable.Cast<object?>().Select(e => e?.ToString() ?? ""));
+            return string.Join(",", enumerable.Cast<object?>().Select(FormatElement));
         }
 
         return value.ToString() ?? "";
     }
+
+    private static string FormatElement(object? element) => element switch
+    {
+        null => "",
+        string str => str,
+        IFormattable f => f.ToString(null, CultureInfo.InvariantCulture) ?? "",
+        _ => element.ToString() ?? ""
+    };
 }