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>

memory/project_linux_graphical.md

@@ -12,9 +12,10 @@ Linux gameplay or renderer fork.
 
 Canonical seams:
 
-- `AcDream.Runtime.Platform.ApplicationPathSet` owns XDG/Windows config, data,
-  cache, plugin, screenshot, and diagnostic paths for graphical and headless
-  hosts.
+- `AcDream.Platform.ApplicationPathSet` (moved out of Runtime by Campaign LA
+  LA0, 2026-08-14) owns XDG/Windows config, data, cache, plugin, screenshot,
+  and diagnostic paths for graphical and headless hosts — and now for the
+  external launcher, which references only `AcDream.Platform`.
 - `AcDream.App.Platform.GraphicalHostPlatformServices` owns one startup OS,
   architecture, RID, native-dependency manifest, path set, and pacing factory.
 - `PlatformFramePacingWaiterFactory` selects the existing Windows