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

DeepDrftPublic.Client/Services/MixVisualizerZoomState.cs

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+namespace DeepDrftPublic.Client.Services;
+
+/// <summary>
+/// Holds the Mix visualizer's zoom (visible time-span in seconds) for the lifetime of the WASM app
+/// instance. Scoped in DI, so it lives across SPA navigations within one listening session — open a
+/// second mix and the slider keeps where you left it — but a fresh page load (F5) constructs a new
+/// instance, resetting to the default. That matches the spec's "persist within session, reset on
+/// fresh load" without any cookie/localStorage round-trip (see phase-9-mix-visualizer-redesign §B).
+/// </summary>
+public sealed class MixVisualizerZoomState
+{
+    /// <summary>
+    /// Default opening window. Mirrors <c>DEFAULT_VISIBLE_SECONDS</c> in MixVisualizer.ts; keep the
+    /// two in sync (the TS owns the rendering anchors, this owns the C#-side session default).
+    /// </summary>
+    public const double DefaultVisibleSeconds = 10.0;
+
+    /// <summary>Visible time-span in seconds. Survives navigation; resets on fresh page load.</summary>
+    public double VisibleSeconds { get; set; } = DefaultVisibleSeconds;
+}