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>

src/AcDream.App/AcDream.App.csproj

@@ -58,6 +58,11 @@
     <ProjectReference Include="..\AcDream.Core\AcDream.Core.csproj" />
     <ProjectReference Include="..\AcDream.Core.Net\AcDream.Core.Net.csproj" />
     <ProjectReference Include="..\AcDream.Content\AcDream.Content.csproj" />
+    <!-- Campaign LA LA0 review finding 6: App consumes AcDream.Platform
+         types directly (GraphicalHostPlatformServices, Program, GameWindow),
+         so the reference is declared explicitly per this file's convention
+         rather than ridden transitively through Runtime. -->
+    <ProjectReference Include="..\AcDream.Platform\AcDream.Platform.csproj" />
     <ProjectReference Include="..\AcDream.UI.Abstractions\AcDream.UI.Abstractions.csproj" />
   </ItemGroup>
   <ItemGroup>