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>

src/AcDream.App/RuntimeOptions.cs

@@ -59,7 +59,6 @@ public sealed record RuntimeOptions(
     float FogEndMultiplier,
     ResidencyBudgetOptions ResidencyBudgets,
     StreamingWorkBudgetOptions StreamingWorkBudgets,
-    RenderBackendKind RenderBackend,
     string? VulkanDeviceOverride,
     string? VulkanForcedUnsupportedFeature,
     bool VulkanCapabilityProbe,
@@ -147,10 +146,6 @@ public sealed record RuntimeOptions(
             FogEndMultiplier:    TryParseFloat(env("ACDREAM_FOG_END_MULT")) ?? 0.95f,
             ResidencyBudgets:    ResidencyBudgetOptions.Parse(env),
             StreamingWorkBudgets: StreamingWorkBudgetOptions.Parse(env),
-            // Campaign V slice V10 flipped this. Unset, empty, or any
-            // unrecognised value now means Vulkan: it is the shipping backend,
-            // and a typo must never silently start the backend V11 deletes.
-            RenderBackend:       ParseRenderBackend(env("ACDREAM_RENDER_BACKEND")),
             // Physical-device override, matched as a decimal index first and then
             // as a case-insensitive device-name substring. Recorded verbatim in
             // graphical-capabilities-vulkan.json whether or not it matched.
@@ -178,29 +173,6 @@ public sealed record RuntimeOptions(
                 TryParseNonNegativeInt(env("ACDREAM_VULKAN_PROBE_FRAMES")) ?? 0);
     }
 
-    /// <summary>
-    /// Startup backend request. Campaign V slice V10 inverted this: Vulkan is
-    /// the default, and only the explicit escape-hatch tokens <c>gl</c> and
-    /// <c>opengl</c> select OpenGL. Everything else — unset, <c>vulkan</c>, or a
-    /// typo — is Vulkan.
-    /// </summary>
-    /// <remarks>
-    /// The polarity of the typo case is deliberate and it flipped with the
-    /// default. Before V10 an unrecognised token had to land on GL, because
-    /// Vulkan was dark and a typo must never silently start a backend that
-    /// cannot draw. After V10 an unrecognised token must land on Vulkan for the
-    /// same reason read the other way: GL is the retiring backend that slice V11
-    /// deletes, so a typo must never silently pin a process to it. <c>opengl</c>
-    /// is honoured alongside <c>gl</c> because the escape hatch exists to be
-    /// found, and the failure mode of a near-miss spelling here is a process
-    /// that quietly is not on the backend the operator asked to fall back to.
-    /// </remarks>
-    private static RenderBackendKind ParseRenderBackend(string? value)
-        => string.Equals(value, "gl", StringComparison.OrdinalIgnoreCase)
-            || string.Equals(value, "opengl", StringComparison.OrdinalIgnoreCase)
-            ? RenderBackendKind.Gl
-            : RenderBackendKind.Vulkan;
-
     /// <summary>True iff live-mode credentials are present and valid for connecting.</summary>
     public bool HasLiveCredentials =>
         LiveMode && !string.IsNullOrEmpty(LiveUser) && !string.IsNullOrEmpty(LivePass);