feat(audience-sdk): add EnableMobileAttribution + SKAdNetworkIds config (SDK-305)

Entry-point ticket for the mobile attribution stretch work. Adds two
opt-in surfaces; downstream tickets (SDK-306/308/309/315) wire them up:

- AudienceConfig.EnableMobileAttribution (default false): runtime gate
  for whether attribution data is collected and emitted on game_launch.
- AudienceConfig.SKAdNetworkIds: optional list, read by the iOS
  post-processor at build time when the scripting define is set.

XML doc on EnableMobileAttribution documents the two-gate model: studios
opt in via both the AUDIENCE_MOBILE_ATTRIBUTION scripting define and
this runtime flag. Without the define, no AD_ID permission, no native
attribution code, NSPrivacyTracking false; with define + flag false,
native code compiled in but no data collected; with both, full flow.

Author: nattb8

src/Packages/Audience/Runtime/AudienceConfig.cs

@@ -49,6 +49,44 @@ public class AudienceConfig
         /// </summary>
         public bool Debug { get; set; } = false;
 
+        /// <summary>
+        /// Opts the build into mobile install-attribution signals (iOS ATT /
+        /// IDFA / SKAdNetwork, Android Advertising ID / Install Referrer).
+        /// </summary>
+        /// <remarks>
+        /// Two gates control attribution. Both must be set for any attribution
+        /// data to ship.
+        ///
+        /// Build-time gate: the <c>AUDIENCE_MOBILE_ATTRIBUTION</c> scripting
+        /// define (Player Settings → Other Settings → Scripting Define
+        /// Symbols). It controls whether the AD_ID Android permission is
+        /// injected into the manifest, which iOS Privacy Manifest variant is
+        /// selected (<c>NSPrivacyTracking</c> true vs false), and whether the
+        /// native ATT / IDFA / GAID / Install Referrer code paths are
+        /// compiled into the binary.
+        ///
+        /// Runtime gate: this flag. With the define set but the flag false,
+        /// native code is compiled in but no attribution data is collected.
+        /// With both set, the SDK emits attribution signals on
+        /// <c>game_launch</c>.
+        ///
+        /// A studio that never wants attribution sets neither and ships a
+        /// clean binary with no AD_ID permission, <c>NSPrivacyTracking</c>
+        /// false, and no native attribution code.
+        /// </remarks>
+        public bool EnableMobileAttribution { get; set; } = false;
+
+        /// <summary>
+        /// Optional list of SKAdNetwork IDs to inject into the iOS
+        /// <c>Info.plist</c> at build time.
+        /// </summary>
+        /// <remarks>
+        /// Read by the iOS post-processor only when the
+        /// <c>AUDIENCE_MOBILE_ATTRIBUTION</c> scripting define is set. Ignored
+        /// otherwise. Has no effect on Android.
+        /// </remarks>
+        public string[]? SKAdNetworkIds { get; set; }
+
         /// <summary>
         /// Interval between automatic flushes to the backend, in seconds.
         /// </summary>

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

@@ -0,0 +1,41 @@
+#nullable enable
+
+using NUnit.Framework;
+
+namespace Immutable.Audience.Tests
+{
+    [TestFixture]
+    internal class AudienceConfigTests
+    {
+        [Test]
+        public void EnableMobileAttribution_DefaultsToFalse()
+        {
+            // Default-off matters: a studio that never opts in must ship a
+            // clean binary, no AD_ID permission, NSPrivacyTracking false.
+            var config = new AudienceConfig();
+            Assert.IsFalse(config.EnableMobileAttribution);
+        }
+
+        [Test]
+        public void SKAdNetworkIds_DefaultsToNull()
+        {
+            var config = new AudienceConfig();
+            Assert.IsNull(config.SKAdNetworkIds);
+        }
+
+        [Test]
+        public void EnableMobileAttribution_RoundTrips()
+        {
+            var config = new AudienceConfig { EnableMobileAttribution = true };
+            Assert.IsTrue(config.EnableMobileAttribution);
+        }
+
+        [Test]
+        public void SKAdNetworkIds_RoundTrips()
+        {
+            var ids = new[] { "abc123.skadnetwork", "def456.skadnetwork" };
+            var config = new AudienceConfig { SKAdNetworkIds = ids };
+            Assert.AreSame(ids, config.SKAdNetworkIds);
+        }
+    }
+}