feat: complete legacy UI and playout parity migration

src/MBN_STOCK_WEBVIEW.Playout/Configuration/LegacyParityStartupCompositionResolver.cs

@@ -0,0 +1,52 @@
+using MBN_STOCK_WEBVIEW.Core.Playout.Scenes;
+
+namespace MBN_STOCK_WEBVIEW.Playout.Configuration;
+
+public sealed record LegacyParityStartupCompositionResolution(
+    LegacySceneCueCompositionOptions Composition,
+    string WarningMessage)
+{
+    public bool HasWarning => WarningMessage.Length > 0;
+}
+
+/// <summary>
+/// Resolves the startup-only MainForm background behavior used by LegacyParityApp.
+/// An explicit local file remains authoritative; only an absent file enables the
+/// validated legacy 기본.vrv fallback.
+/// </summary>
+public static class LegacyParityStartupCompositionResolver
+{
+    public const string MissingLegacyDefaultWarning =
+        "신뢰 배경 폴더에서 기본.vrv를 확인할 수 없어 배경없음으로 시작했습니다.";
+
+    public static LegacyParityStartupCompositionResolution Resolve(
+        PlayoutOptions options,
+        bool explicitLocalConfigurationExists)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        var configured = PlayoutSceneCompositionFactory.Create(options);
+        if (explicitLocalConfigurationExists ||
+            configured.BackgroundKind != LegacySceneBackgroundKind.None)
+        {
+            return new LegacyParityStartupCompositionResolution(configured, string.Empty);
+        }
+
+        try
+        {
+            return new LegacyParityStartupCompositionResolution(
+                PlayoutSceneCompositionFactory.CreateLegacyDefault(
+                    options,
+                    configured.FadeDuration),
+                string.Empty);
+        }
+        catch (PlayoutConfigurationException)
+        {
+            // Missing/invalid external media is not an application-startup failure.
+            // Keep the already validated None composition and expose one bounded,
+            // path-free warning through the native state projection.
+            return new LegacyParityStartupCompositionResolution(
+                configured,
+                MissingLegacyDefaultWarning);
+        }
+    }
+}