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>

stemroller/qml/StemRoller/Elevation.qml

@@ -0,0 +1,21 @@
+import QtQuick
+
+// Static 9-patch shadow: no runtime blur, one textured quad.
+BorderImage {
+    id: shadow
+    property int level: 2
+    property int radius: 8
+    readonly property real sigma: Theme.shadowSigma[level]
+    readonly property int m: Math.ceil(sigma * 3) + 1
+    readonly property int dy: Theme.shadowDy[level]
+    visible: level > 0
+    z: -1
+    anchors.fill: parent
+    anchors.leftMargin: -m
+    anchors.rightMargin: -m
+    anchors.topMargin: -m + dy
+    anchors.bottomMargin: -m - dy
+    border { left: m + radius; right: m + radius; top: m + radius; bottom: m + radius }
+    source: "image://fx/shadow/" + sigma + "/" + Theme.shadowAlpha[level] + "/" + radius
+    cache: true
+}