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>

.github/workflows/headless-portability.yml

@@ -5,6 +5,7 @@ on:
     paths:
       - ".github/workflows/headless-portability.yml"
       - "AcDream.slnx"
+      - "src/AcDream.Platform/**"
       - "src/AcDream.Core/**"
       - "src/AcDream.Core.Net/**"
       - "src/AcDream.Content/**"
@@ -13,6 +14,7 @@ on:
       - "src/AcDream.Headless/**"
       - "src/AcDream.App/**"
       - "src/AcDream.UI.Abstractions/**"
+      - "tests/AcDream.Platform.Tests/**"
       - "tests/AcDream.Core.Tests/**"
       - "tests/AcDream.Core.Net.Tests/**"
       - "tests/AcDream.Content.Tests/**"
@@ -26,6 +28,7 @@ on:
     paths:
       - ".github/workflows/headless-portability.yml"
       - "AcDream.slnx"
+      - "src/AcDream.Platform/**"
       - "src/AcDream.Core/**"
       - "src/AcDream.Core.Net/**"
       - "src/AcDream.Content/**"
@@ -34,6 +37,7 @@ on:
       - "src/AcDream.Headless/**"
       - "src/AcDream.App/**"
       - "src/AcDream.UI.Abstractions/**"
+      - "tests/AcDream.Platform.Tests/**"
       - "tests/AcDream.Core.Tests/**"
       - "tests/AcDream.Core.Net.Tests/**"
       - "tests/AcDream.Content.Tests/**"
@@ -78,6 +82,7 @@ jobs:
         shell: pwsh
         run: |
           $projects = @(
+            "src/AcDream.Platform/AcDream.Platform.csproj",
             "src/AcDream.Plugin.Abstractions/AcDream.Plugin.Abstractions.csproj",
             "src/AcDream.Core/AcDream.Core.csproj",
             "src/AcDream.Core.Net/AcDream.Core.Net.csproj",
@@ -98,6 +103,7 @@ jobs:
         shell: pwsh
         run: |
           $projects = @(
+            "tests/AcDream.Platform.Tests/AcDream.Platform.Tests.csproj",
             "tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj",
             "tests/AcDream.Content.Tests/AcDream.Content.Tests.csproj",
             "tests/AcDream.Runtime.Tests/AcDream.Runtime.Tests.csproj",