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/build.gradle.kts

@@ -23,7 +23,7 @@ android {
 
     buildTypes {
         getByName("release") {
-            consumerProguardFiles("consumer-rules.pro")
+            consumerProguardFiles("consumer-rules.keep")
         }
     }
 }