feat(platform): Campaign LA LA0 — extract ApplicationPathSet to AcDream.Platform

The launcher (LA3/LA4) needs the XDG/Windows path contract
(ApplicationPathSet/IApplicationPathEnvironment) without pulling in any
gameplay assembly. Move it out of AcDream.Runtime into a new BCL-only
AcDream.Platform project so the launcher-side Launcher.Core project can
reference it directly per the campaign plan (docs/plans/2026-08-14-launcher-campaign.md,
LA0). Namespace renamed AcDream.Runtime.Platform -> AcDream.Platform;
code is otherwise byte-identical (no logic changes).

AcDream.Runtime now carries a ProjectReference to AcDream.Platform and
re-exports it transitively, so App and Headless keep resolving the type
without a direct reference and K0's Headless single-ProjectReference
guard (HeadlessAssemblyReferencesOnlyTheRuntimeProject) stands unchanged.
The sibling Runtime dependency-boundary guard
(RuntimeProjectDeclaresOnlyApprovedProjectDependencies) does assert
Runtime's own project-reference set, so it needed a deliberate,
documented addition of AcDream.Platform to its expected list.

Moved tests/AcDream.Runtime.Tests/Platform/ApplicationPathSetTests.cs to
a new tests/AcDream.Platform.Tests/ project (namespace
AcDream.Platform.Tests) referencing only AcDream.Platform. Registered
both new projects in AcDream.slnx.

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

src/AcDream.App/Program.cs

@@ -3,7 +3,7 @@ using AcDream.App.Plugins;
 using AcDream.App.Platform;
 using AcDream.App.Rendering;
 using AcDream.Core.Plugins;
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 using Serilog;
 
 GraphicalHostPlatformServices graphicalPlatform =