phase(N.4): WbMeshAdapter stub + IWbMeshAdapter interface

Stub adapter that validates constructor args and exposes the public
shape (IncrementRefCount / DecrementRefCount / GetRenderData / Dispose).
Real ObjectMeshManager init is deferred to Task 9 — for now methods
no-op so call sites can wire the adapter without behavioral effect.

IWbMeshAdapter interface enables mocking in subsequent tasks
(LandblockSpawnAdapter tests in Task 11 need it).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/AcDream.App/Rendering/Wb/IWbMeshAdapter.cs

@@ -0,0 +1,12 @@
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Mockable interface over <see cref="WbMeshAdapter"/> so adapters that
+/// drive ref-count lifecycle (e.g. LandblockSpawnAdapter, EntitySpawnAdapter)
+/// can be unit-tested without a real WB pipeline behind them.
+/// </summary>
+public interface IWbMeshAdapter
+{
+    void IncrementRefCount(ulong id);
+    void DecrementRefCount(ulong id);
+}