feat(render): Vulkan campaign V11 step 2 — delete the OpenGL backend

Vulkan is the sole, user-signed-off backend (V10 landed) and step 1
already removed ImGui/Studio/DevTools. This step deletes the GL
rendering backend itself: every Gpu/Gl/** implementation, the Wb
ManagedGL*/GLHelpers/GLSLShader/GLStateScope/RenderStateCache/
BindlessSupport family, Shader/ShaderProgramConstruction/SamplerCache,
RenderBootstrap, and RenderFrameGlStateController.

GameWindow.cs's Run()/CreateGraphics()/CreateBackbufferReader()/
OnLoad() collapse to their Vulkan-only arm; GameWindowGraphics loses
its OpenGlGameWindowGraphics subclass. RuntimeOptions.RenderBackend and
RenderBackendKind (incl. the Gl member of GpuBackendKind) are gone —
there is nothing left to select between. The five world-draw dual-arm
renderers (WbDrawDispatcher, EnvCellRenderer, TerrainModernRenderer,
ParticleRenderer, SkyRenderer) and the composition roots
(WorldRenderComposition, HostInputCameraComposition,
LivePresentationComposition, FrameRootComposition) collapse to their
RHI-only arm. GL-only diagnostic properties with a live external reader
(DynamicBufferCount and friends) simplify to a documented `=> 0`/no-op
rather than disappearing, since the reader is out of this commit's
scope.

A few GL-flavored mechanisms turned out to be backend-neutral once
isolated: GlConstructionCleanupLedger is renamed
ResourceConstructionCleanupLedger (exception-chain walking has nothing
to do with GL), and GlfwNativePlatformProbe moved out of the otherwise
GL-only GraphicalCapabilityRecord.cs into
GraphicalWindowBackendSelection.cs before the rest of that file was
deleted.

Test files with no surviving subject are deleted outright
(GraphicalCapabilityRequirementsTests, ShaderProgramConstructionTests,
PortalDepthShaderParityTests, TextureCacheBindlessTests,
TextRendererFailureSafetyTests, ClipFrameUploadTests, every
Gpu/Gl/*Tests, GlTextureOwnershipTests, RenderFrameGlStateControllerTests);
others get their dead GL-only members trimmed while their live
assertions stay (ClipFrameLayoutTests' MeshClipSsboBinding check now
reads GpuBindingModel.StorageClipRegions, the same binding index under
its new backend-neutral name; GpuResourceRetirementTransactionTests
drops its OpenGLGraphicsDevice-subclassing test double and the two GL
queue tests it existed for). EnvCellRendererTests' construction helper
now builds a real ObjectMeshManager via VulkanMeshPipelineDevice
instead of passing null through a null-forgiving operator, since the
RHI constructor never tolerated a null mesh manager and the old GL
constructor (which did) is gone.

Deferred to the next two steps, deliberately not touched here: the
Silk.NET.OpenGL/.Extensions.ARB package references, IMeshPipelineDevice.Gl
(WbMeshAdapter's GL? threading stays in place), Chorizite.Core's stale
csproj comment (the package itself is still load-bearing —
TextureFormat and friends are used well beyond the deleted
ManagedGLUniformBuffer), and the CI/gate scripts.

Build: `dotnet build AcDream.slnx -c Release` — 0 warnings, 0 errors.
Tests: full-solution `dotnet test` green across every project
(App.Tests 3937/3940 + 3 skips, Core.Tests 3296/3298 + 2 skips, all
others 100%); the 2 App.Tests names that flake under full-suite
parallel execution (#250-family, documented pre-existing) pass in
isolation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/RuntimeOptionsTests.cs

@@ -475,75 +475,6 @@ public sealed class RuntimeOptionsTests
         Assert.Throws<ArgumentNullException>(() => RuntimeOptions.Parse(AnyDatDir, null!));
     }
 
-    /// <summary>
-    /// Campaign V slice V10 flipped the default: Vulkan is the shipping backend
-    /// and an unset variable must select it. Awaiting the user's cutover
-    /// sign-off; the one-line rollback restores GL here and in
-    /// <c>RuntimeOptions.ParseRenderBackend</c> together.
-    /// </summary>
-    [Fact]
-    public void RenderBackend_DefaultsToVulkan()
-    {
-        Assert.Equal(
-            RenderBackendKind.Vulkan,
-            RuntimeOptions.Parse(AnyDatDir, EmptyEnv()).RenderBackend);
-    }
-
-    [Theory]
-    [InlineData("vulkan")]
-    [InlineData("Vulkan")]
-    [InlineData("VULKAN")]
-    public void RenderBackend_SelectsVulkanCaseInsensitively(string value)
-    {
-        Assert.Equal(
-            RenderBackendKind.Vulkan,
-            RuntimeOptions.Parse(
-                AnyDatDir,
-                Env(new() { ["ACDREAM_RENDER_BACKEND"] = value })).RenderBackend);
-    }
-
-    /// <summary>
-    /// The V10 escape hatch, and the whole of it. Both spellings are honoured
-    /// because a near-miss here strands the operator on the backend they asked
-    /// to leave.
-    /// </summary>
-    [Theory]
-    [InlineData("gl")]
-    [InlineData("GL")]
-    [InlineData("Gl")]
-    [InlineData("opengl")]
-    [InlineData("OpenGL")]
-    public void RenderBackend_SelectsGlOnlyForTheEscapeHatchTokens(string value)
-    {
-        Assert.Equal(
-            RenderBackendKind.Gl,
-            RuntimeOptions.Parse(
-                AnyDatDir,
-                Env(new() { ["ACDREAM_RENDER_BACKEND"] = value })).RenderBackend);
-    }
-
-    /// <summary>
-    /// The mirror image of the pre-V10 rule. A typo used to have to land on GL
-    /// because Vulkan was dark; it now has to land on Vulkan because GL is the
-    /// backend slice V11 deletes, and a misspelled fallback that silently works
-    /// is how a process ends up pinned to it.
-    /// </summary>
-    [Theory]
-    [InlineData("")]
-    [InlineData("vulcan")]
-    [InlineData("vk")]
-    [InlineData(" vulkan")]
-    [InlineData(" gl")]
-    [InlineData("ogl")]
-    public void RenderBackend_AnythingElseStaysOnVulkan(string value)
-    {
-        Assert.Equal(
-            RenderBackendKind.Vulkan,
-            RuntimeOptions.Parse(
-                AnyDatDir,
-                Env(new() { ["ACDREAM_RENDER_BACKEND"] = value })).RenderBackend);
-    }
-
     [Fact]
     public void VulkanDeviceOverride_IsNullWhenUnsetOrEmpty()
     {