refactor(audience-sdk): name JSON timestamp and round-trip number formats

Replaces the bare format-spec strings "o" and "R" passed to DateTime and float / double ToString
calls with named constants on Constants so the wire-shape requirement reads as a contract rather
than a mystery character.

- Constants.cs: adds IsoTimestampFormat ("o") and RoundTripNumberFormat ("R") with comments noting
  the backend schema requirement and the round-trip preservation guarantee.
- MessageBuilder.cs: BuildBase eventTimestamp uses Constants.IsoTimestampFormat.
- Json.cs: float and double serialisation use Constants.RoundTripNumberFormat.
- AudienceSample.UI.cs: log-row export timestamp uses Constants.IsoTimestampFormat.

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Scripts/AudienceSample.UI.cs

@@ -780,7 +780,7 @@ private VisualElement BuildLogRow(LogEntry entry)
         private static string FormatLogEntry(LogEntry entry, bool singleLine)
         {
             var sb = new StringBuilder()
-                .Append(entry.Timestamp.ToString("o", CultureInfo.InvariantCulture))
+                .Append(entry.Timestamp.ToString(Constants.IsoTimestampFormat, CultureInfo.InvariantCulture))
                 .Append(" [").Append(entry.Source == LogSource.Sdk ? "SDK" : "APP").Append("] ")
                 .Append(entry.Label);
             if (!string.IsNullOrEmpty(entry.Body))

src/Packages/Audience/Runtime/Core/Constants.cs

@@ -17,6 +17,12 @@ internal static class Constants
         internal const int MaxBatchSize = 100;
         internal const int StaleEventDays = 30;
         internal const int MaxFieldLength = 256; // Backend schema limit.
+
+        // Timestamp format the backend wants on every event.
+        internal const string IsoTimestampFormat = "o";
+
+        // Format that lets numbers survive a JSON round-trip unchanged.
+        internal const string RoundTripNumberFormat = "R";
         internal const int ControlPlaneRequestTimeoutSeconds = 30;
 
         internal const string LibraryName = "com.immutable.audience";

src/Packages/Audience/Runtime/Events/MessageBuilder.cs

@@ -79,7 +79,7 @@ private static Dictionary<string, object> BuildBase(string type, string packageV
             {
                 [MessageFields.Type] = type,
                 ["messageId"] = Guid.NewGuid().ToString(),
-                ["eventTimestamp"] = DateTime.UtcNow.ToString("o"),
+                ["eventTimestamp"] = DateTime.UtcNow.ToString(Constants.IsoTimestampFormat),
                 ["context"] = new Dictionary<string, object>
                 {
                     ["library"] = Constants.LibraryName,

src/Packages/Audience/Runtime/Utility/Json.cs

@@ -60,14 +60,14 @@ private static void WriteValue(StringBuilder sb, object? value, int indent, int
                 if (float.IsNaN(f) || float.IsInfinity(f))
                     sb.Append("null");
                 else
-                    sb.Append(f.ToString("R", CultureInfo.InvariantCulture));
+                    sb.Append(f.ToString(Constants.RoundTripNumberFormat, CultureInfo.InvariantCulture));
             }
             else if (value is double d)
             {
                 if (double.IsNaN(d) || double.IsInfinity(d))
                     sb.Append("null");
                 else
-                    sb.Append(d.ToString("R", CultureInfo.InvariantCulture));
+                    sb.Append(d.ToString(Constants.RoundTripNumberFormat, CultureInfo.InvariantCulture));
             }
             else if (value is decimal dec)
             {