fix(platform): Campaign LA LA0 review fixes — CI Linux lanes, arch doc, self-guard

Opus dual-lens review of cb6502c8 passed with six findings; this lands
the fix round:
1. headless-portability.yml: AcDream.Platform src/tests join both path
   triggers and the presentation-free build/test arrays — the moved XDG
   tests run on ubuntu-latest again (they had fallen out of every Linux
   lane).
2. acdream-architecture.md: AcDream.Platform gets its own layer block;
   Runtime may-reference clause updated (the guard changed in cb6502c8,
   its human-readable twin had not).
3. PlatformDependencyBoundaryTests: the BCL-only contract (zero
   project/package references) is now enforced, not just observed.
4. memory/project_linux_graphical.md canonical seam renamed.
5. Plan LA0 recon corrected: the K0 Headless guard was never the guard
   needing amendment (it asserts Headless own refs); Runtime own-refs
   guard was — the commit did the right thing, the plan text now says so.
6. App declares its AcDream.Platform reference explicitly per its own
   convention instead of riding transitivity.

Platform.Tests: 4 passed (3 moved + the new guard).

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

tests/AcDream.Platform.Tests/PlatformDependencyBoundaryTests.cs

@@ -0,0 +1,62 @@
+using System.Runtime.CompilerServices;
+using System.Xml.Linq;
+
+namespace AcDream.Platform.Tests;
+
+// Campaign LA LA0 review finding 3: AcDream.Platform's entire premise is
+// being the BCL-only assembly the external launcher can reference without
+// pulling any gameplay code. That contract is what this guard enforces —
+// the csproj must declare zero ProjectReference and zero PackageReference
+// entries, forever, in the same spirit as Runtime's and Headless's
+// dependency-boundary guards.
+public sealed class PlatformDependencyBoundaryTests
+{
+    [Fact]
+    public void PlatformProjectDeclaresNoProjectOrPackageDependencies()
+    {
+        var repositoryRoot = FindRepositoryRoot();
+        var projectPath = Path.Combine(
+            repositoryRoot,
+            "src",
+            "AcDream.Platform",
+            "AcDream.Platform.csproj");
+        var project = XDocument.Load(projectPath);
+
+        Assert.Empty(project.Descendants("ProjectReference"));
+        Assert.Empty(project.Descendants("PackageReference"));
+    }
+
+    private static string FindRepositoryRoot(
+        [CallerFilePath] string sourcePath = "")
+    {
+        string[] starts =
+        {
+            Path.GetDirectoryName(sourcePath) ?? string.Empty,
+            Directory.GetCurrentDirectory(),
+            AppContext.BaseDirectory,
+        };
+        foreach (string start in starts)
+        {
+            if (string.IsNullOrEmpty(start))
+            {
+                continue;
+            }
+
+            var directory = new DirectoryInfo(start);
+            while (directory is not null)
+            {
+                if (File.Exists(Path.Combine(
+                        directory.FullName,
+                        "AcDream.slnx")))
+                {
+                    return directory.FullName;
+                }
+
+                directory = directory.Parent;
+            }
+        }
+
+        throw new DirectoryNotFoundException(
+            "Could not find AcDream.slnx above the source, working, or output directory.");
+    }
+}