feat(render #53): EntityClassificationCache skeleton + first test

Adds CachedBatch, EntityCacheEntry, and EntityClassificationCache with
just TryGet (returns false on empty). The skeleton compiles and the first
test (TryGet_EmptyCache_ReturnsFalse) passes. Subsequent tasks add
Populate, InvalidateEntity, InvalidateLandblock, and the dispatcher
integration. Per spec design Section 6.1.

Note: CachedBatch / EntityCacheEntry / EntityClassificationCache are
internal (not public as the plan snippet showed). Their members
transitively reference the internal GroupKey type, so promoting them to
public produces CS0051 inconsistent-accessibility errors. The cache is
dispatcher-internal coordination state anyway, and the AcDream.App
csproj already exposes internals to AcDream.Core.Tests via
InternalsVisibleTo, so the test sees everything it needs.

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

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

@@ -0,0 +1,39 @@
+using System.Numerics;
+
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Per-(entity, partIdx, batchIdx) classification result, stored flat inside
+/// <see cref="EntityCacheEntry.Batches"/>. For Setup multi-part MeshRefs each
+/// subPart contributes its own <see cref="CachedBatch"/> entries, with
+/// <see cref="RestPose"/> already containing the
+/// <c>subPart.PartTransform * meshRef.PartTransform</c> product.
+///
+/// Accessibility: <c>internal</c> because <see cref="GroupKey"/> is
+/// <c>internal</c> and shows up in this struct's constructor / <c>Deconstruct</c>
+/// signature. The cache itself is dispatcher-internal coordination state;
+/// <see cref="InternalsVisibleTo"/> on <c>AcDream.App</c> exposes the type to
+/// <c>AcDream.Core.Tests</c>.
+/// </summary>
+internal readonly record struct CachedBatch(
+    GroupKey Key,
+    ulong BindlessTextureHandle,
+    Matrix4x4 RestPose);
+
+/// <summary>
+/// One entity's cached classification. <see cref="Batches"/> is flat across
+/// (partIdx, batchIdx) and ordered as <c>WbDrawDispatcher.ClassifyBatches</c>
+/// produced them. <see cref="LandblockHint"/> lets
+/// <see cref="EntityClassificationCache.InvalidateLandblock"/> sweep entries
+/// efficiently when a landblock demotes or unloads.
+///
+/// Accessibility: <c>internal</c> for the same reason as <see cref="CachedBatch"/>
+/// — its <see cref="Batches"/> property is <c>CachedBatch[]</c>, which
+/// transitively involves <see cref="GroupKey"/>.
+/// </summary>
+internal sealed class EntityCacheEntry
+{
+    public required uint EntityId { get; init; }
+    public required uint LandblockHint { get; init; }
+    public required CachedBatch[] Batches { get; init; }
+}