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>

docs/architecture/acdream-architecture.md

@@ -266,14 +266,21 @@ src/
     World/
       RuntimeWorldEnvironmentState.cs -> canonical calendar/time/weather owner
       RuntimeWorldTransitState.cs -> canonical reveal generation/readiness owner
-    Platform/
-      ApplicationPathSet.cs -> shared BCL-only XDG/Windows config, data,
-                               cache, plugin, screenshot, and diagnostic paths
     RuntimeGenerationReset.cs -> one retryable canonical-generation reset
     -> Slice J complete; graphical and no-window hosts share one GameRuntime
-    -> may reference Core, Core.Net, Content, and Plugin.Abstractions only
+    -> may reference Core, Core.Net, Content, Plugin.Abstractions, and
+       Platform only
     -> must never reference App, UI, Silk.NET, OpenAL, or Arch
 
+  AcDream.Platform/          BCL-only portable path contract (Campaign LA LA0)
+    ApplicationPathSet.cs    -> shared XDG/Windows config, data, cache,
+                                plugin, screenshot, and diagnostic paths
+    -> zero project/package references (guarded by
+       tests/AcDream.Platform.Tests/PlatformDependencyBoundaryTests.cs);
+       Runtime references it and re-exports transitively to App/Headless;
+       the external launcher (AcDream.Launcher.Core) references ONLY this
+       project from the game solution
+
   AcDream.Headless/          Linux/Windows no-window production host
     Program.cs               -> CLI entry only
     Configuration/           -> strict versioned process/session config