REDI-91: MVVM contrast screen (:feature:about:presentation)

Add a deliberately small MVVM About screen: AboutViewModel exposes a StateFlow plus
plain public methods (onToggleMvvmNote) with NO Action sealed type and NO Event channel
— the explicit contrast to the app's MVI screens. AboutScreen collects state and calls
the VM method directly; links open via LocalUriHandler. Static showcase copy lives in
the VM as state to demonstrate the StateFlow-as-content shape. aboutGraph + @Serializable
AboutRoute + aboutPresentationModule wire it in. A code comment and the README explain
why this is MVVM and when each pattern fits.

feature/about/presentation/build.gradle.kts

@@ -1,5 +1,7 @@
 plugins {
     alias(libs.plugins.architecture.android.feature)
+    // For @Serializable type-safe navigation routes.
+    alias(libs.plugins.architecture.kotlinx.serialization)
 }
 
 // MVVM contrast screen (StateFlow + plain VM methods, no Action/Event funnel). Static content,