feat(visualizer): add stereo scopes

Add a theme-aware goniometer and pitch-locked oscilloscope to the large dock and immersive focus modes.

Gate scope DSP and scenegraph updates by actual panel visibility so hidden modes remain idle, including while shader scenes cover immersive panels.

cx

crates/qbz-qt/qml/shell/SpectrumBand.qml

@@ -1,6 +1,6 @@
 // SpectrumBand — the compact 42px spectrum strip above the Large-NPB cover.
 // Split out of SidebarNowPlayingDock.qml: the dock owns the layout arithmetic
-// and the cover, this file owns the three render modes and their motion.
+// and the cover, this file owns the five render modes and their motion.
 // QML port of the `spectrum := Rectangle` block in
 // crates/qbz-ui/ui/shell/SidebarNowPlayingDock.slint:122-183 (VBar / WaveCol).
 //
@@ -88,6 +88,7 @@
 import QtQuick
 import QtQuick.Window
 import com.blitzfc.qbz
+import "../controls"
 import "../theme"
 
 Rectangle {
@@ -436,7 +437,29 @@ Rectangle {
         }
     }
 
-    // Click the band → cycle Bars -> Waveform -> Energy (persisted).
+    // Modes 3/4 — real-time scopes. Each Loader owns a single scene-graph
+    // line strip; when inactive the item does not exist and Rust also disables
+    // that scope's DSP bit.
+    Loader {
+        anchors.fill: parent
+        active: band.shown && QbzShell.largeSpectrumMode === 3
+        sourceComponent: ScopePanel {
+            compact: true
+            scopeMode: 0
+            traceColor: theme.accent
+        }
+    }
+    Loader {
+        anchors.fill: parent
+        active: band.shown && QbzShell.largeSpectrumMode === 4
+        sourceComponent: ScopePanel {
+            compact: true
+            scopeMode: 1
+            traceColor: theme.accent
+        }
+    }
+
+    // Click the band → cycle all five modes (persisted).
     MouseArea {
         anchors.fill: parent
         cursorShape: Qt.PointingHandCursor