Force MultiThreaded execution mode and remove SingleThread UI option

SingleThread is incompatible with the Android Vulkan surface-readiness
path (VeldridDevice polls SurfaceHandle for up to 5s on the same thread
that publishes it in SingleThread mode -> deadlock -> null Vulkan func
ptr -> SDLThread SI_TKILL crash within ~5s of launch, which matches the
tablet crash report).

The same pattern would also bite iOS Metal drawable attach. On desktop
SingleThread is just slower with no UX benefit. Upstream ppy/osu defaults
to MultiThreaded; we now enforce it on every startup so a stale
framework.ini cannot pin a user into the broken mode.

- OsuGameBase.load: force ExecutionMode = MultiThreaded once on load.
- RendererSettings: drop the Threading Mode dropdown.
- GraphicsSettingsStrings: drop the now-unused ThreadingMode string.
- TestSceneOsuGame: drop TestSwitchThreadExecutionMode (covered behaviour
  is no longer user-reachable) and prune the now-unused usings.

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/cccaf70f-1049-4fde-a21c-cd61a27bf8b2

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game.Tests/Visual/Navigation/TestSceneOsuGame.cs

@@ -8,9 +8,7 @@
 using NUnit.Framework;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
-using osu.Framework.Configuration;
 using osu.Framework.Graphics.Textures;
-using osu.Framework.Platform;
 using osu.Game.Audio;
 using osu.Game.Beatmaps;
 using osu.Game.Configuration;
@@ -104,13 +102,6 @@ public void TestNullRulesetHandled()
             AddAssert("ruleset unchanged", () => ReferenceEquals(Ruleset.Value, ruleset));
         }
 
-        [Test]
-        public void TestSwitchThreadExecutionMode()
-        {
-            AddStep("Change thread mode to multi threaded", () => { Game.Dependencies.Get<FrameworkConfigManager>().SetValue(FrameworkSetting.ExecutionMode, ExecutionMode.MultiThreaded); });
-            AddStep("Change thread mode to single thread", () => { Game.Dependencies.Get<FrameworkConfigManager>().SetValue(FrameworkSetting.ExecutionMode, ExecutionMode.SingleThread); });
-        }
-
         [Test]
         public void TestUnavailableRulesetHandled()
         {

osu.Game/Localisation/GraphicsSettingsStrings.cs

@@ -29,11 +29,6 @@ public static class GraphicsSettingsStrings
         /// </summary>
         public static LocalisableString FrameLimiter => new TranslatableString(getKey(@"frame_limiter"), @"Frame limiter");
 
-        /// <summary>
-        /// "Threading mode"
-        /// </summary>
-        public static LocalisableString ThreadingMode => new TranslatableString(getKey(@"threading_mode"), @"Threading mode");
-
         /// <summary>
         /// "Show FPS"
         /// </summary>

osu.Game/OsuGameBase.cs

@@ -329,6 +329,21 @@ private void load(ReadableKeyCombinationProvider keyCombinationProvider, Framewo
 
             MessageFormatter.WebsiteRootUrl = endpoints.WebsiteUrl;
 
+            // Force the framework execution mode to MultiThreaded on every startup.
+            //
+            // SingleThread used to be exposed in the graphics settings, but it is incompatible
+            // with our Android Vulkan-surface readiness path: VeldridDevice / AndroidGameHost
+            // poll AndroidGameWindow.SurfaceHandle for up to 5s, and in SingleThread mode that
+            // poll runs on the same thread that publishes the surface handle, deadlocking
+            // initialisation until the timeout fires and Vulkan device creation crashes with a
+            // null function pointer (see PR history around the SDLThread/SI_TKILL crash).
+            //
+            // The same risk applies to iOS Metal layer attach (drawable surfaces are also
+            // delivered via the main loop), and on desktop SingleThread is strictly slower with
+            // no UX benefit. Upstream ppy/osu defaults to MultiThreaded; we now enforce it
+            // unconditionally so a stale `framework.ini` cannot pin a user into the broken mode.
+            frameworkConfig.SetValue(FrameworkSetting.ExecutionMode, ExecutionMode.MultiThreaded);
+
             // Initialise localisation
             frameworkLocale = frameworkConfig.GetBindable<string>(FrameworkSetting.Locale);
             frameworkLocale.BindValueChanged(_ => updateLanguage());

osu.Game/Overlays/Settings/Sections/Graphics/RendererSettings.cs

@@ -83,11 +83,6 @@ private void load(FrameworkConfigManager config, OsuConfigManager osuConfig, IDi
                     Keywords = new[] { @"fps", @"framerate" },
                 },
                 customDrawLimitItem,
-                new SettingsItemV2(new FormEnumDropdown<ExecutionMode>
-                {
-                    Caption = GraphicsSettingsStrings.ThreadingMode,
-                    Current = config.GetBindable<ExecutionMode>(FrameworkSetting.ExecutionMode)
-                }),
                 new SettingsItemV2(new FormCheckBox
                 {
                     Caption = GraphicsSettingsStrings.ShowFPS,