fix(audio): fix CAVA audio pipeline and harden bar count handling

CAVA requires even bar counts for stereo output but odd values could
reach the process, causing silent exit code 1. The audioSpectrum QML
binding also used || [] which forced V4 JS conversion, losing the
native QVector<float> type needed by the C++ fast path.

- Enforce even bar counts at all layers (CavaService, KCM setter, slider)
- Move bar count constants to shared Audio::MinBars/MaxBars in constants.h
- Use Binding element with `when` guard for audioSpectrum to avoid
  undefined hitting the slow QVariantList conversion path
- Move exit diagnostics from stateChanged to finished signal per Qt API
- Only warn on non-zero exit code (stderr on exit 0 is normal for CAVA)
- Switch to SeparateChannels to capture CAVA stderr for diagnostics
- Add static_assert for even MinBars/MaxBars at compile time

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: fuddlesworth

kcm/kcm_plasmazones.cpp

@@ -814,8 +814,11 @@ void KCMPlasmaZones::setEnableAudioVisualizer(bool enable)
 
 void KCMPlasmaZones::setAudioSpectrumBarCount(int count)
 {
-    if (m_settings->audioSpectrumBarCount() != count) {
-        m_settings->setAudioSpectrumBarCount(count);
+    // CAVA requires even bar count for stereo output
+    const int even = (count % 2 != 0) ? count + 1 : count;
+    const int clamped = qBound(Audio::MinBars, even, Audio::MaxBars);
+    if (m_settings->audioSpectrumBarCount() != clamped) {
+        m_settings->setAudioSpectrumBarCount(clamped);
         Q_EMIT audioSpectrumBarCountChanged();
         setNeedsSave(true);
     }

kcm/ui/tabs/ZonesTab.qml

@@ -413,9 +413,9 @@ ScrollView {
                         Slider {
                             id: audioBarsSlider
                             Layout.preferredWidth: root.constants.sliderPreferredWidth
-                            from: 16
-                            to: 256
-                            stepSize: 1
+                            from: 16  // Audio::MinBars (src/core/constants.h)
+                            to: 256   // Audio::MaxBars (src/core/constants.h)
+                            stepSize: 2  // CAVA requires even bar count for stereo
                             value: kcm.audioSpectrumBarCount
                             onMoved: kcm.audioSpectrumBarCount = Math.round(value)
                         }

src/core/constants.h

@@ -158,6 +158,14 @@ inline constexpr QLatin1String AutoAssign{"autoAssign"};
 inline constexpr QLatin1String Colors{"colors"};
 }
 
+/**
+ * @brief Audio visualization constants (CAVA)
+ */
+namespace Audio {
+constexpr int MinBars = 16;
+constexpr int MaxBars = 256;
+}
+
 /**
  * @brief D-Bus service constants
  */

src/daemon/cavaservice.cpp

@@ -7,13 +7,14 @@
 #include <QStandardPaths>
 #include <QFileInfo>
 
+#include "../core/constants.h"
 #include "../core/logging.h"
 
 namespace PlasmaZones {
 
 static constexpr int kAsciiMaxRange = 1000; // CAVA ascii_max_range
-static constexpr int kMinBars = 16;
-static constexpr int kMaxBars = 256;
+static_assert(Audio::MinBars % 2 == 0, "Audio::MinBars must be even for CAVA stereo");
+static_assert(Audio::MaxBars % 2 == 0, "Audio::MaxBars must be even for CAVA stereo");
 
 CavaService::CavaService(QObject* parent)
     : QObject(parent)
@@ -52,15 +53,18 @@ void CavaService::start()
                 this, &CavaService::onReadyReadStandardOutput);
         connect(m_process, &QProcess::stateChanged,
                 this, &CavaService::onProcessStateChanged);
+        connect(m_process, &QProcess::finished,
+                this, &CavaService::onProcessFinished);
         connect(m_process, &QProcess::errorOccurred,
                 this, &CavaService::onProcessError);
     }
 
     m_stdoutBuffer.clear();
     m_spectrum.clear();
 
-    // Kurve-style: pass config via stdin, read raw output from stdout
-    m_process->setProcessChannelMode(QProcess::ForwardedErrorChannel);
+    // Kurve-style: pass config via stdin, read raw output from stdout.
+    // Use SeparateChannels so we can capture stderr for error diagnostics.
+    m_process->setProcessChannelMode(QProcess::SeparateChannels);
     m_process->start(QStringLiteral("sh"), QStringList{QStringLiteral("-c"),
         QStringLiteral("exec %1 -p /dev/stdin <<'CAVAEOF'\n%2\nCAVAEOF").arg(cavaPath, m_config)});
 
@@ -90,7 +94,10 @@ bool CavaService::isRunning() const
 
 void CavaService::setBarCount(int count)
 {
-    const int clamped = qBound(kMinBars, count, kMaxBars);
+    // CAVA requires even bar count for stereo output (bars split between L/R channels).
+    // Round to even first, then clamp — ensures we never exceed MaxBars after rounding.
+    int even = (count % 2 != 0) ? count + 1 : count;
+    const int clamped = qBound(Audio::MinBars, even, Audio::MaxBars);
     if (m_barCount != clamped) {
         m_barCount = clamped;
         if (isRunning()) {
@@ -207,6 +214,19 @@ void CavaService::onProcessStateChanged(QProcess::ProcessState state)
     }
 }
 
+void CavaService::onProcessFinished(int exitCode, QProcess::ExitStatus /*exitStatus*/)
+{
+    if (m_stopping || m_pendingRestart) {
+        return;
+    }
+    if (exitCode != 0) {
+        const QByteArray stderrOutput = m_process ? m_process->readAllStandardError().left(500)
+                                                  : QByteArray();
+        qCWarning(lcOverlay) << "CAVA exited with code" << exitCode
+                             << "stderr:" << stderrOutput;
+    }
+}
+
 void CavaService::onProcessError(QProcess::ProcessError error)
 {
     // Suppress errors from intentional stop() or restartAsync() — SIGTERM causes QProcess::Crashed

src/daemon/cavaservice.h

@@ -63,6 +63,7 @@ class CavaService : public QObject
     void buildConfig();
     void onReadyReadStandardOutput();
     void onProcessStateChanged(QProcess::ProcessState state);
+    void onProcessFinished(int exitCode, QProcess::ExitStatus exitStatus);
     void onProcessError(QProcess::ProcessError error);
     void restartAsync();
     static QString detectAudioMethod();

src/shared/ZoneShaderRenderer.qml

@@ -44,15 +44,23 @@ Item {
         iResolution: root.safeConfig.iResolution || Qt.size(width, height)
         iMouse: root.safeConfig.iMouse || Qt.point(0, 0)
 
-        audioSpectrum: root.safeConfig.audioSpectrum || []
-
         onStatusChanged: {
             if (status === ZoneShaderItem.Error) {
                 root.shaderError(errorLog)
             }
         }
     }
 
+    // Use Binding with `when` guard to avoid passing undefined to the C++ setter
+    // when config is null. Without this, undefined hits the slow QVariantList path
+    // in setAudioSpectrumVariant instead of preserving QVector<float> type identity.
+    Binding {
+        target: zoneShaderItem
+        property: "audioSpectrum"
+        value: root.safeConfig.audioSpectrum
+        when: root.safeConfig.audioSpectrum !== undefined
+    }
+
     Binding {
         target: zoneShaderItem
         property: "labelsTexture"