Keep the Showkase codegen constructor in the designsystem module

The rule only kept the class, but ShowkaseBrowserActivity instantiates it with
Class.forName(...).getDeclaredConstructor().newInstance(), so R8 was free to
strip the constructor. Also drop the AGP template comments and rename the file
to .keep, for consistency with app/proguard.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

libraries/designsystem/consumer-rules.keep

@@ -0,0 +1,5 @@
+# Showkase generates this class, and Showkase's ShowkaseBrowserActivity loads it reflectively with
+# Class.forName(...).getDeclaredConstructor().newInstance(), so its constructor must be kept too.
+-keep class io.element.android.libraries.designsystem.showkase.DesignSystemShowkaseRootModuleCodegen {
+    <init>();
+}