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);
+}

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

@@ -0,0 +1,72 @@
+using System;
+using DatReaderWriter;
+using Microsoft.Extensions.Logging;
+using Silk.NET.OpenGL;
+
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Single seam between acdream and WB's render pipeline. Owns the
+/// <c>ObjectMeshManager</c> instance (when fully initialized) and exposes
+/// a stable acdream-shaped API so the rest of the renderer doesn't need
+/// to know about WB's types directly.
+///
+/// <para>
+/// <b>Phase N.4 staging:</b> currently a stub. Real <c>ObjectMeshManager</c>
+/// + <c>OpenGLGraphicsDevice</c> initialization is added in Task 9 once
+/// the dat-reader adapter (Task 6) lands. Until then, methods no-op so
+/// call sites can wire the adapter without behavioral effect when the
+/// <see cref="WbFoundationFlag.IsEnabled"/> flag is on.
+/// </para>
+/// </summary>
+public sealed class WbMeshAdapter : IDisposable, IWbMeshAdapter
+{
+    // _meshManager and _graphicsDevice will be wired in Task 9 once
+    // WbDatReaderAdapter (Task 6) lands. For now, both are null and all
+    // methods no-op.
+    // private ObjectMeshManager? _meshManager;
+    // private OpenGLGraphicsDevice? _graphicsDevice;
+    private bool _disposed;
+
+    public WbMeshAdapter(GL gl, DatCollection dats, ILogger<WbMeshAdapter> logger)
+    {
+        ArgumentNullException.ThrowIfNull(gl);
+        ArgumentNullException.ThrowIfNull(dats);
+        ArgumentNullException.ThrowIfNull(logger);
+
+        // TODO(N.4 Task 9): construct OpenGLGraphicsDevice and ObjectMeshManager
+        // once WbDatReaderAdapter (Task 6) is available to bridge our DatCollection
+        // to WB's IDatReaderWriter.
+    }
+
+    private WbMeshAdapter()
+    {
+        // Uninitialized constructor — only for tests / for cases where the
+        // flag is off and the caller wants a Dispose-safe no-op instance.
+    }
+
+    /// <summary>Test/init helper — produces a Dispose-safe instance with no
+    /// underlying mesh manager. Public methods are all no-ops.</summary>
+    public static WbMeshAdapter CreateUninitialized() => new();
+
+    /// <summary>Returns null until Task 9 wires up the real mesh manager.</summary>
+    public object? GetRenderData(ulong id) => null;
+
+    public void IncrementRefCount(ulong id)
+    {
+        // No-op until Task 9.
+    }
+
+    public void DecrementRefCount(ulong id)
+    {
+        // No-op until Task 9.
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        // _meshManager?.Dispose();
+        // _graphicsDevice?.Dispose();
+    }
+}

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);
+    }
+}