StemRoller: desktop stem separator, Android stems player, and server daemons

- stemroller/: PySide6/QML desktop app and player for separating music into stems
- android/: Plex music player that plays per-stem mixes, with Android Auto support,
  a graphic + parametric (Equalizer APO-style) EQ, and offline downloads
- server/: stemserve (serves stems to the app) and stemd (separates the library on
  the server's GPU, with "Stem Now" priority requests)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/proguard-rules.pro

@@ -0,0 +1,10 @@
+# kotlinx.serialization keeps its generated serializers via its own consumer rules.
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.**
+-keep,includedescriptorclasses class com.stemroller.mobile.**$$serializer { *; }
+-keepclassmembers class com.stemroller.mobile.** {
+    *** Companion;
+}
+-keepclasseswithmembers class com.stemroller.mobile.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}