feat(public): scrolling Canvas 2D Mix visualizer — windowed, playback-coupled, zoomable, read-only (8.K W2)

DeepDrftPublic.Client/Controls/MixZoomMapping.cs

@@ -0,0 +1,36 @@
+namespace DeepDrftPublic.Client.Controls;
+
+/// <summary>
+/// Pure mapping between the Mix visualizer's zoom slider position [0, 1] and the visible time-span in
+/// seconds. The span range is wide (0.333 s … 30 s, ~90×), so the mapping is logarithmic — equal
+/// slider travel changes the span by an equal *ratio*, which feels even to the hand. Slider
+/// orientation: fraction 0 = most zoomed-out (longest span), fraction 1 = most zoomed-in (the
+/// 0.333 s quarter-note-@-180-BPM anchor). Extracted from the component so the math is unit-testable.
+/// </summary>
+public static class MixZoomMapping
+{
+    /// <summary>Shortest span (max zoom): one quarter note at 180 BPM = 60/180 s. Hard anchor.</summary>
+    public const double MinVisibleSeconds = 60.0 / 180.0;
+
+    /// <summary>Longest span (min zoom). Tunable.</summary>
+    public const double MaxVisibleSeconds = 30.0;
+
+    /// <summary>Slider position [0, 1] -> visible seconds. 0 = zoomed out, 1 = zoomed in.</summary>
+    public static double FractionToSeconds(double fraction)
+    {
+        fraction = Math.Clamp(fraction, 0, 1);
+        var logMax = Math.Log(MaxVisibleSeconds);
+        var logMin = Math.Log(MinVisibleSeconds);
+        // Interpolate in log space from out (frac 0 -> logMax) to in (frac 1 -> logMin).
+        return Math.Exp(logMax + (logMin - logMax) * fraction);
+    }
+
+    /// <summary>Visible seconds -> slider position [0, 1]. Inverse of <see cref="FractionToSeconds"/>.</summary>
+    public static double SecondsToFraction(double seconds)
+    {
+        seconds = Math.Clamp(seconds, MinVisibleSeconds, MaxVisibleSeconds);
+        var logMax = Math.Log(MaxVisibleSeconds);
+        var logMin = Math.Log(MinVisibleSeconds);
+        return (logMax - Math.Log(seconds)) / (logMax - logMin);
+    }
+}