arch(runtime): establish presentation-independent boundary

Create the dependency-only Runtime project before moving any gameplay owner, enforce its direct, transitive, source, and load-time closure, and pin coherent lifetime-group extraction plus retryable teardown ordering. No production behavior changes in J0.

Validated by the four focused Runtime boundary tests, a zero-error Release solution build, and 8,406 passing Release tests with five pre-existing skips.

Co-authored-by: Codex <codex@openai.com>

src/AcDream.Runtime/RuntimeAssemblyMarker.cs

@@ -0,0 +1,10 @@
+namespace AcDream.Runtime;
+
+/// <summary>
+/// Identifies the presentation-independent runtime assembly.
+/// </summary>
+/// <remarks>
+/// Slice J0 establishes only the dependency boundary. Runtime owners move here
+/// in coherent lifetime groups during the later Slice J steps.
+/// </remarks>
+internal static class RuntimeAssemblyMarker;