feat(audience): add Unity integration layer and pure-C# core partition (SDK-147)

Splits the package into a core asmdef (pure C#, buildable headless
under dotnet) and a Unity sub-asmdef (engine-referencing). The
Unity layer has always belonged here; the fence on the core was
always correct; landing both together makes the partition
enforceable at Unity-compile time.

- Runtime/Unity/AudienceUnityHooks.cs: RuntimeInitializeOnLoadMethod
  wiring. Flushes on pause/quit via Application hooks. Installs
  Debug.Log as Log.Writer for SDK diagnostics.
- Runtime/Unity/com.immutable.audience.unity.asmdef: sibling
  sub-asmdef claiming Runtime/Unity/. Excludes unsupported
  platforms (mobile, consoles) per v1 scope (Windows / macOS /
  Linux). Referenced from the core asmdef's sibling - not the
  other way around, keeping the dependency direction clean.
- Runtime/com.immutable.audience.asmdef: noEngineReferences
  flipped to true. The core has always been pure C# (proven by
  the sibling Audience.Runtime.csproj that compiles the same tree
  headless), but the flag shipped as Unity's Editor-default false.
  Now that the Unity sub-asmdef exists to hold UnityEngine-using
  code, flipping the fence makes the guarantee enforceable at
  Unity-compile time - stray `using UnityEngine` in Core/,
  Events/, Transport/, or Utility/ now breaks the Unity build,
  matching what dotnet already rejects.
- Audience.Runtime.csproj: comment now cross-references the
  asmdef flag as the primary portability fence and the Compile
  Remove as the sibling check for the headless dotnet build.
  Prevents a future cleanup from removing one of the two halves.

Author: ImmutableJeffrey

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

@@ -11,6 +11,12 @@
     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.
+
+    Portability enforcement: com.immutable.audience.asmdef sets
+    noEngineReferences: true, which makes stray `using UnityEngine` in Core/,
+    Events/, Transport/, Utility/ fail to compile inside Unity. This
+    Compile Remove is the sibling check for the headless dotnet build.
+    Keep both.
   -->
   <ItemGroup>
     <Compile Remove="Unity/**/*.cs" />

src/Packages/Audience/Runtime/Unity/AudienceUnityHooks.cs

@@ -0,0 +1,22 @@
+using UnityEngine;
+
+namespace Immutable.Audience.Unity
+{
+    internal static class AudienceUnityHooks
+    {
+        [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]
+        private static void Install()
+        {
+            // Clear surviving statics before re-wiring in case "disable domain reload" kept them alive.
+            ImmutableAudience.ResetState();
+
+            // -= then += so repeat SubsystemRegistration cycles don't stack subscriptions.
+            Application.quitting -= ImmutableAudience.Shutdown;
+            Application.quitting += ImmutableAudience.Shutdown;
+
+            ImmutableAudience.DefaultPersistentDataPathProvider = () => Application.persistentDataPath;
+
+            if (Log.Writer == null) Log.Writer = Debug.Log;
+        }
+    }
+}

src/Packages/Audience/Runtime/Unity/com.immutable.audience.unity.asmdef

@@ -0,0 +1,14 @@
+{
+    "name": "Immutable.Audience.Unity",
+    "rootNamespace": "Immutable.Audience.Unity",
+    "references": ["Immutable.Audience.Runtime"],
+    "includePlatforms": ["Editor","LinuxStandalone64","macOSStandalone","WindowsStandalone64"],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": true,
+    "defineConstraints": [],
+    "versionDefines": [],
+    "noEngineReferences": false
+}

src/Packages/Audience/Runtime/com.immutable.audience.asmdef

@@ -10,5 +10,5 @@
     "autoReferenced": true,
     "defineConstraints": [],
     "versionDefines": [],
-    "noEngineReferences": false
+    "noEngineReferences": true
 }