build(audience): add IL2CPP link.xml for Unity ship

Plan §6.6 requires a link.xml at the package root so IL2CPP's
managed-code stripping doesn't remove types the SDK reaches only
at runtime. Preserves the full Immutable.Audience.Runtime assembly
and names the System.Net.Http pipeline pieces used by
HttpTransport / SetConsent / DeleteData. System.IO.Compression
entries are harmless when the gzip scripting define is off.

Author: ImmutableJeffrey

src/Packages/Audience/link.xml

@@ -0,0 +1,31 @@
+<!--
+IL2CPP link.xml for com.immutable.audience.
+
+Preserves types that IL2CPP might otherwise strip under aggressive
+managed-code stripping. The SDK's own types are kept via full-assembly
+preservation on Immutable.Audience.Runtime; BCL assemblies list only the
+namespaces/types actually reached at runtime (HttpClient pipeline, gzip
+when the scripting define is on).
+
+Plan §6.6 calls this file out explicitly. Regenerate if the transport or
+serialisation stack gains a new framework dependency.
+-->
+<linker>
+  <assembly fullname="Immutable.Audience.Runtime" preserve="all" />
+
+  <assembly fullname="System.Net.Http">
+    <type fullname="System.Net.Http.HttpClient" preserve="all" />
+    <type fullname="System.Net.Http.HttpClientHandler" preserve="all" />
+    <type fullname="System.Net.Http.HttpMessageHandler" preserve="all" />
+    <type fullname="System.Net.Http.HttpRequestMessage" preserve="all" />
+    <type fullname="System.Net.Http.HttpResponseMessage" preserve="all" />
+    <type fullname="System.Net.Http.StringContent" preserve="all" />
+    <type fullname="System.Net.Http.ByteArrayContent" preserve="all" />
+    <type fullname="System.Net.Http.Headers.MediaTypeHeaderValue" preserve="all" />
+  </assembly>
+
+  <assembly fullname="System.IO.Compression">
+    <type fullname="System.IO.Compression.GZipStream" preserve="all" />
+    <type fullname="System.IO.Compression.CompressionLevel" preserve="all" />
+  </assembly>
+</linker>