test(vulkan): isolate the hardware witness lane

Tag the production offscreen witness as Lane=Vulkan, exclude capability-owned tests from the portable Release filter, and give lavapipe an explicit trait-only invocation after the portable Vulkan contracts.

Document the lane and pin witness, portable-filter, and workflow ownership without initializing Vulkan.

Mutations performed and restored:

1. Removed the witness trait: VulkanLaneOwnershipContractTests first failed Assert.Matches because the Lane=Vulkan/Fact/method pattern was absent.

2. Removed Lane!=Vulkan from the default filter: the contract first failed Assert.Contains, not found Lane!=Vulkan.

3. Weakened the dedicated invocation to Lane!=Vulkan: the contract first failed Assert.Contains, not found --filter Lane=Vulkan in the hardware step.

4. Narrowed the dedicated invocation with FullyQualifiedName: the contract first failed Assert.DoesNotContain because FullyQualifiedName was present at position 231.

tests/AcDream.App.Tests/Rendering/Gpu/Vk/MeshModernSharedIndexOffscreenTests.cs

@@ -19,6 +19,7 @@ public sealed unsafe class MeshModernSharedIndexOffscreenTests
     private const uint Prefix = 2;
     private static readonly object VulkanLock = new();
 
+    [Trait("Lane", "Vulkan")]
     [Fact]
     public void CommittedProductionOrdinaryShader_RendersLocalSidecarsAtNonzeroTransformPrefix()
     {

tests/AcDream.App.Tests/Rendering/Gpu/Vk/VulkanLaneOwnershipContractTests.cs

@@ -0,0 +1,84 @@
+using System.Text.RegularExpressions;
+
+namespace AcDream.App.Tests.Rendering.Gpu.Vk;
+
+/// <summary>
+/// Keeps the capability-owned pixel witness out of the portable gate while
+/// requiring the lavapipe job to execute the complete trait-owned lane.
+/// This contract reads repository text only and never initializes Vulkan.
+/// </summary>
+public sealed class VulkanLaneOwnershipContractTests
+{
+    private const string WitnessMethod =
+        "CommittedProductionOrdinaryShader_RendersLocalSidecarsAtNonzeroTransformPrefix";
+
+    [Fact]
+    public void HardwareWitness_PortableFilterAndLavapipeOwnerStayAligned()
+    {
+        string root = RepositoryRoot();
+        string witness = File.ReadAllText(Path.Combine(
+            root,
+            "tests", "AcDream.App.Tests", "Rendering", "Gpu", "Vk",
+            "MeshModernSharedIndexOffscreenTests.cs"));
+        Assert.Matches(
+            new Regex(
+                "\\[Trait\\(\"Lane\", \"Vulkan\"\\)\\]\\s*"
+                + "\\[Fact\\]\\s*public void " + WitnessMethod + "\\(",
+                RegexOptions.CultureInvariant),
+            witness);
+        Assert.Equal(1, Count(witness, "[Trait(\"Lane\", \"Vulkan\")]"));
+
+        string releaseGate = File.ReadAllText(Path.Combine(root, "tools", "run-release-gate.ps1"));
+        Match defaultFilter = Regex.Match(
+            releaseGate,
+            "\\[string\\]\\$TestFilter\\s*=\\s*'(?<filter>[^']+)'",
+            RegexOptions.CultureInvariant);
+        Assert.True(defaultFilter.Success, "Could not locate the portable Release gate's default filter.");
+        Assert.Contains("Lane!=Vulkan", defaultFilter.Groups["filter"].Value, StringComparison.Ordinal);
+        Assert.Equal(1, Count(defaultFilter.Groups["filter"].Value, "Lane!=Vulkan"));
+
+        string workflow = File.ReadAllText(Path.Combine(
+            root, ".github", "workflows", "headless-portability.yml"));
+        int install = workflow.IndexOf("- name: Install lavapipe, the Vulkan loader and Xvfb", StringComparison.Ordinal);
+        int portable = workflow.IndexOf("- name: Test the Vulkan backend's platform-independent decisions", StringComparison.Ordinal);
+        int hardware = workflow.IndexOf("- name: Test the Vulkan hardware lane on lavapipe", StringComparison.Ordinal);
+        Assert.True(install >= 0 && install < portable && portable < hardware);
+
+        string portableStep = Step(workflow, portable);
+        Assert.Contains(
+            "--filter \"FullyQualifiedName~AcDream.App.Tests.Rendering.Gpu.Vk&Lane!=Vulkan\"",
+            portableStep,
+            StringComparison.Ordinal);
+
+        string hardwareStep = Step(workflow, hardware);
+        Assert.DoesNotContain("FullyQualifiedName", hardwareStep, StringComparison.Ordinal);
+        Assert.DoesNotContain(WitnessMethod, hardwareStep, StringComparison.Ordinal);
+        Assert.Contains("--filter \"Lane=Vulkan\"", hardwareStep, StringComparison.Ordinal);
+    }
+
+    private static string Step(string workflow, int start)
+    {
+        int next = workflow.IndexOf("      - name:", start + 1, StringComparison.Ordinal);
+        return next < 0 ? workflow[start..] : workflow[start..next];
+    }
+
+    private static int Count(string value, string needle)
+    {
+        int count = 0;
+        for (int index = 0; (index = value.IndexOf(needle, index, StringComparison.Ordinal)) >= 0;)
+        {
+            count++;
+            index += needle.Length;
+        }
+        return count;
+    }
+
+    private static string RepositoryRoot()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "AcDream.slnx")))
+            directory = directory.Parent;
+        return directory?.FullName
+            ?? throw new InvalidOperationException("Could not locate the repository root.");
+    }
+}