test(A.5 T21): lock in depth-write attribution per translucency kind

Per Phase A.5 spec §4.9.3 audit: opaque + ClipMap pass uses
DepthMask(true); AlphaBlend / Additive / InvAlpha pass uses
DepthMask(false), restored after. Audit confirmed correct in
WbDrawDispatcher.Draw. IsOpaquePublic shim already present.

Add WbDispatcherDepthMaskTests: 5-case Theory that pins the partition
so future regressions surface immediately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,39 @@
+using AcDream.App.Rendering.Wb;
+using AcDream.Core.Meshing;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+/// <summary>
+/// A.5 T21: lock in the depth-write attribution per translucency kind.
+/// <para>
+/// <c>WbDrawDispatcher.Draw</c> uses a two-pass structure:
+/// <list type="bullet">
+///   <item>Opaque pass — <c>DepthMask(true)</c>: writes depth so that
+///     later transparent geometry sorts correctly against solid surfaces.</item>
+///   <item>Transparent pass — <c>DepthMask(false)</c>: reads depth but
+///     does NOT write it, so alpha-blended surfaces don't occlude each
+///     other by Z-fighting.</item>
+/// </list>
+/// The partition that decides which pass a batch enters is
+/// <see cref="WbDrawDispatcher.IsOpaquePublic"/>:
+/// <c>Opaque</c> and <c>ClipMap</c> go to the opaque pass (depth write);
+/// <c>AlphaBlend</c>, <c>Additive</c>, <c>InvAlpha</c> go to the
+/// transparent pass (no depth write).
+/// </para>
+/// </summary>
+public sealed class WbDispatcherDepthMaskTests
+{
+    [Theory]
+    [InlineData(TranslucencyKind.Opaque,     true)]   // opaque pass — depth write
+    [InlineData(TranslucencyKind.ClipMap,    true)]   // foliage — depth write (binary alpha / A2C)
+    [InlineData(TranslucencyKind.AlphaBlend, false)]  // transparent — no depth write
+    [InlineData(TranslucencyKind.Additive,   false)]
+    [InlineData(TranslucencyKind.InvAlpha,   false)]
+    public void IsOpaquePartition_ImpliesDepthWriteAttribution(
+        TranslucencyKind kind, bool expectsDepthWrite)
+    {
+        bool isOpaque = WbDrawDispatcher.IsOpaquePublic(kind);
+        Assert.Equal(expectsDepthWrite, isOpaque);
+    }
+}