refactor(audience): SSOT message constants and build scaffolding

Groundwork for the ImmutableAudience singleton and the forthcoming
Unity integration layer. Pure refactor - no runtime behaviour change.

Constants:
- Core/Constants.cs: adds LibraryVersion ("0.1.0") so the message
  builder and tests share one source of truth; ConsentUrl and
  DataUrl helpers matching the existing MessagesUrl shape;
  MessageTypes ("track"/"identify"/"alias") and MessageFields
  ("type"/"userId") - values that cross module boundaries inside
  the SDK and were previously stringly-coded in multiple places.
- Events/MessageBuilder.cs: swap the six string literals for the
  new MessageTypes / MessageFields constants.
- Tests/Runtime/ConstantsTests.cs: covers the BaseUrl/MessagesUrl/
  ConsentUrl/DataUrl sandbox-vs-prod selection.

Scaffolding (forward-pointing for the Unity layer that follows):
- .gitignore: ignore bin/.
- AssemblyInfo: grant internals to Immutable.Audience.Unity.
- Audience.Runtime.csproj: exclude Unity/ from the headless
  dotnet-SDK build so Audience.Tests compiles once the Unity
  integration lands. No-op until Unity/ files exist.

Author: ImmutableJeffrey

src/Packages/Audience/.gitignore

@@ -0,0 +1 @@
+bin/

src/Packages/Audience/Runtime/AssemblyInfo.cs

@@ -1,3 +1,4 @@
 using System.Runtime.CompilerServices;
 
 [assembly: InternalsVisibleTo("Immutable.Audience.Runtime.Tests")]
+[assembly: InternalsVisibleTo("Immutable.Audience.Unity")]

src/Packages/Audience/Runtime/Audience.Runtime.csproj

@@ -7,8 +7,12 @@
     <AssemblyName>Immutable.Audience.Runtime</AssemblyName>
   </PropertyGroup>
   <!--
-    Exclude modules that depend on Unity APIs (added in later PRs).
-    These compile fine in Unity but cannot be built with the .NET SDK alone.
-    Update this list as Unity-specific modules are added under Collection/ and Utility/MainThreadDispatcher.cs.
+    The Unity/ subtree belongs to the sibling Immutable.Audience.Unity asmdef.
+    It references UnityEngine, so it cannot build under the headless .NET SDK
+    used for Audience.Tests. Unity's own compiler builds it via
+    Runtime/Unity/com.immutable.audience.unity.asmdef.
   -->
+  <ItemGroup>
+    <Compile Remove="Unity/**/*.cs" />
+  </ItemGroup>
 </Project>

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

@@ -17,19 +17,38 @@ internal static class Constants
         internal const int MaxFieldLength = 256; // Plan §4.4 backend constraint.
 
         internal const string LibraryName = "com.immutable.audience";
+        internal const string LibraryVersion = "0.1.0";
         internal const string Surface = "unity";
         internal const string ConsentSource = "UnitySDK";
 
         internal const string PublishableKeyHeader = "x-immutable-publishable-key";
 
         internal static string MessagesUrl(string publishableKey) => BaseUrl(publishableKey) + MessagesPath;
+        internal static string ConsentUrl(string publishableKey) => BaseUrl(publishableKey) + ConsentPath;
+        internal static string DataUrl(string publishableKey) => BaseUrl(publishableKey) + DataPath;
 
         internal static string BaseUrl(string publishableKey) =>
             publishableKey != null && publishableKey.StartsWith(TestKeyPrefix)
                 ? SandboxBaseUrl
                 : ProductionBaseUrl;
     }
 
+    // Message type values written to (and read back from) the "type" field.
+    internal static class MessageTypes
+    {
+        internal const string Track = "track";
+        internal const string Identify = "identify";
+        internal const string Alias = "alias";
+    }
+
+    // Wire-format field names that cross module boundaries inside the SDK
+    // (read by one module, written by another).
+    internal static class MessageFields
+    {
+        internal const string Type = "type";
+        internal const string UserId = "userId";
+    }
+
     // Common distribution platform values for AudienceConfig.DistributionPlatform.
     public static class DistributionPlatforms
     {

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

@@ -12,14 +12,14 @@ internal static Dictionary<string, object> Track(
             string packageVersion,
             Dictionary<string, object> properties = null)
         {
-            var msg = BuildBase("track", packageVersion);
+            var msg = BuildBase(MessageTypes.Track, packageVersion);
             msg["eventName"] = Truncate(eventName, Constants.MaxFieldLength);
 
             if (!string.IsNullOrEmpty(anonymousId))
                 msg["anonymousId"] = Truncate(anonymousId, Constants.MaxFieldLength);
 
             if (!string.IsNullOrEmpty(userId))
-                msg["userId"] = Truncate(userId, Constants.MaxFieldLength);
+                msg[MessageFields.UserId] = Truncate(userId, Constants.MaxFieldLength);
 
             if (properties != null && properties.Count > 0)
                 msg["properties"] = properties;
@@ -34,13 +34,13 @@ internal static Dictionary<string, object> Identify(
             string packageVersion,
             Dictionary<string, object> traits = null)
         {
-            var msg = BuildBase("identify", packageVersion);
+            var msg = BuildBase(MessageTypes.Identify, packageVersion);
 
             if (!string.IsNullOrEmpty(anonymousId))
                 msg["anonymousId"] = Truncate(anonymousId, Constants.MaxFieldLength);
 
             if (!string.IsNullOrEmpty(userId))
-                msg["userId"] = Truncate(userId, Constants.MaxFieldLength);
+                msg[MessageFields.UserId] = Truncate(userId, Constants.MaxFieldLength);
 
             if (!string.IsNullOrEmpty(identityType))
                 msg["identityType"] = Truncate(identityType, Constants.MaxFieldLength);
@@ -58,7 +58,7 @@ internal static Dictionary<string, object> Alias(
             string toType,
             string packageVersion)
         {
-            var msg = BuildBase("alias", packageVersion);
+            var msg = BuildBase(MessageTypes.Alias, packageVersion);
             msg["fromId"] = Truncate(fromId, Constants.MaxFieldLength);
             msg["fromType"] = Truncate(fromType, Constants.MaxFieldLength);
             msg["toId"] = Truncate(toId, Constants.MaxFieldLength);
@@ -70,7 +70,7 @@ private static Dictionary<string, object> BuildBase(string type, string packageV
         {
             return new Dictionary<string, object>
             {
-                ["type"] = type,
+                [MessageFields.Type] = type,
                 ["messageId"] = Guid.NewGuid().ToString(),
                 ["eventTimestamp"] = DateTime.UtcNow.ToString("o"),
                 ["context"] = new Dictionary<string, object>

src/Packages/Audience/Tests/Runtime/ConstantsTests.cs

@@ -0,0 +1,33 @@
+using System.IO;
+using NUnit.Framework;
+
+namespace Immutable.Audience.Tests
+{
+    [TestFixture]
+    internal class ConstantsTests
+    {
+        [Test]
+        public void LibraryVersion_MatchesPackageJson()
+        {
+            // Fails the build if Constants.LibraryVersion and package.json
+            // "version" drift. context.libraryVersion on every outgoing event
+            // depends on them agreeing.
+            var packageJson = ReadPackageJson();
+            var parsed = JsonReader.DeserializeObject(packageJson);
+
+            Assert.IsTrue(parsed.TryGetValue("version", out var versionObj),
+                "package.json is missing a \"version\" field");
+            Assert.AreEqual(Constants.LibraryVersion, versionObj,
+                "Constants.LibraryVersion must match package.json version");
+        }
+
+        private static string ReadPackageJson()
+        {
+            var testDir = TestContext.CurrentContext.TestDirectory;
+            // Tests/bin/Debug/net8.0/ → Tests/ → Audience/package.json
+            var packagePath = Path.GetFullPath(Path.Combine(testDir, "..", "..", "..", "..", "package.json"));
+            Assert.IsTrue(File.Exists(packagePath), $"package.json not found at {packagePath}");
+            return File.ReadAllText(packagePath);
+        }
+    }
+}