Add Psytrance Visualizer macOS app with Metal rendering

A complete audio-reactive visualizer for psytrance music featuring:

Audio Analysis (DSPEngine):
- FFT spectrum analysis via Accelerate/vDSP
- 64-band Mel spectrogram
- Sub-bass energy extraction (<100Hz)
- Automatic sidechain pump detection
- Harmonic-to-Noise ratio (HNR) calculation
- Peak/transient detection

8 Visualization Modes (Metal Shaders):
1. FFT Classic - Frequency spectrum bars with glow
2. Mel Spectrogram - Waterfall display
3. Sub-Bass - Pulsating rings
4. Sidechain Pump - Breathing zoom effect
5. Harmonic/Noise - Geometric vs chaotic particles
6. Mandelbrot - Audio-reactive fractal zoom
7. Tunnel Warp - Infinite tunnel with distortion
8. DMT Geometry - Sacred geometry patterns

Features:
- Selectable audio input device (BlackHole support)
- Configurable buffer size (512/1024)
- Reactivity slider for visual intensity
- Auto-hiding control panel
- Fullscreen support with keyboard shortcuts (1-8, F, ESC)
- Persistent settings via UserDefaults
- Psytrance-inspired neon/UV color palette

PsytranceVisualizer/Models/AppSettings.swift

@@ -0,0 +1,90 @@
+//
+//  AppSettings.swift
+//  PsytranceVisualizer
+//
+//  Persistent application settings
+//
+
+import Foundation
+
+/// Application settings that are persisted between sessions
+struct AppSettings: Codable {
+    /// Selected audio input device UID
+    var selectedAudioDeviceUID: String?
+
+    /// Audio buffer size (512 or 1024 samples)
+    var bufferSize: Int
+
+    /// Last used visualization mode (1-8)
+    var lastVisualizationMode: Int
+
+    /// Reactivity slider value (0.0 - 1.0)
+    var reactivity: Float
+
+    /// Whether app was in fullscreen mode
+    var isFullscreen: Bool
+
+    /// Last window frame (for restoration)
+    var windowFrame: CodableRect?
+
+    /// Volume/gain adjustment
+    var inputGain: Float
+
+    /// Whether to show FPS counter
+    var showFPS: Bool
+
+    /// Default settings
+    static var `default`: AppSettings {
+        AppSettings(
+            selectedAudioDeviceUID: nil,
+            bufferSize: 1024,
+            lastVisualizationMode: 1,
+            reactivity: 0.5,
+            isFullscreen: false,
+            windowFrame: nil,
+            inputGain: 1.0,
+            showFPS: false
+        )
+    }
+
+    /// Available buffer sizes
+    static let availableBufferSizes = [512, 1024]
+
+    /// Validates and clamps settings to valid ranges
+    mutating func validate() {
+        // Clamp buffer size to valid options
+        if !AppSettings.availableBufferSizes.contains(bufferSize) {
+            bufferSize = 1024
+        }
+
+        // Clamp visualization mode
+        if lastVisualizationMode < 1 || lastVisualizationMode > 8 {
+            lastVisualizationMode = 1
+        }
+
+        // Clamp reactivity
+        reactivity = max(0.0, min(1.0, reactivity))
+
+        // Clamp input gain
+        inputGain = max(0.0, min(2.0, inputGain))
+    }
+}
+
+/// Codable wrapper for CGRect
+struct CodableRect: Codable {
+    var x: Double
+    var y: Double
+    var width: Double
+    var height: Double
+
+    init(from rect: CGRect) {
+        self.x = Double(rect.origin.x)
+        self.y = Double(rect.origin.y)
+        self.width = Double(rect.size.width)
+        self.height = Double(rect.size.height)
+    }
+
+    var cgRect: CGRect {
+        CGRect(x: x, y: y, width: width, height: height)
+    }
+}