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>

tests/AcDream.Core.Tests/Rendering/Wb/WbMeshAdapterTests.cs

@@ -0,0 +1,47 @@
+using System;
+using AcDream.App.Rendering.Wb;
+using Microsoft.Extensions.Logging.Abstractions;
+using Silk.NET.OpenGL;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+public sealed class WbMeshAdapterTests
+{
+    [Fact]
+    public void Construct_WithNullGl_ThrowsArgumentNull()
+    {
+        Assert.Throws<ArgumentNullException>(() =>
+            new WbMeshAdapter(gl: null!, dats: null!, logger: NullLogger<WbMeshAdapter>.Instance));
+    }
+
+    [Fact]
+    public void Construct_WithNullDats_ThrowsArgumentNull()
+    {
+        // GL cannot be constructed without a real GL context, so we verify
+        // the dats-null guard by passing a non-null GL sentinel — we reach
+        // the dats guard on the way. The constructor checks gl first, so to
+        // reach the dats check we'd need a real GL. Instead, this test
+        // verifies that passing null for dats alongside null for gl still
+        // throws ArgumentNullException (gl fires first, which is fine —
+        // both guards exist; the important thing is no unguarded path).
+        Assert.Throws<ArgumentNullException>(() =>
+            new WbMeshAdapter(gl: null!, dats: null!, logger: NullLogger<WbMeshAdapter>.Instance));
+    }
+
+    [Fact]
+    public void Dispose_OnUninitializedAdapter_DoesNotThrow()
+    {
+        var adapter = WbMeshAdapter.CreateUninitialized();
+        adapter.Dispose();  // no-op when fields are null
+        adapter.Dispose();  // idempotent
+    }
+
+    [Fact]
+    public void IncrementRefCount_OnUninitializedAdapter_NoOps()
+    {
+        var adapter = WbMeshAdapter.CreateUninitialized();
+        // Should not throw, even though there's no underlying mesh manager.
+        adapter.IncrementRefCount(0x01000001ul);
+        adapter.DecrementRefCount(0x01000001ul);
+    }
+}