phase(N.4) Tasks 16+18+19: AnimatedEntityState + AnimPartChange + HiddenParts

Per-entity render state for the per-instance rendering tier
(server-spawned characters / creatures / equipped items). Holds:
- partGfxObjOverrides: Dictionary<int, ulong> — AnimPartChange swaps
  (e.g. wielding a weapon replaces a hand-part's GfxObj).
- hiddenMask: ulong — HiddenParts bitmask. Bit i set hides part i.
- AnimationSequencer reference — N.4 doesn't touch the sequencer;
  this just exposes it for the draw dispatcher.

Public API: HideParts / IsPartHidden / SetPartOverride /
TryGetPartOverride / ResolvePartGfxObj. Bounds-checked
(partIdx < 0 or >= 64 → IsPartHidden returns false).

Twelve tests covering the type, the AnimPartChange resolution helper,
and the HiddenParts bitmask edge cases (theories for 0b0/0b1/MSB/all-ones,
plus negative-index + out-of-range guards).

Consumed by Task 17's EntitySpawnAdapter (creates one per CreateObject)
and Task 22's WbDrawDispatcher (reads via per-part draw loop).

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

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

@@ -0,0 +1,45 @@
+using AcDream.App.Rendering.Wb;
+using AcDream.Core.Physics;
+using DatReaderWriter.DBObjs;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+public sealed class AnimatedEntityStateTests
+{
+    [Fact]
+    public void DefaultState_HasNoOverridesAndNoHiddenParts()
+    {
+        var state = MakeState();
+
+        Assert.False(state.IsPartHidden(0));
+        Assert.False(state.IsPartHidden(63));
+        Assert.False(state.TryGetPartOverride(0, out _));
+    }
+
+    [Fact]
+    public void Sequencer_AccessibleAsProperty()
+    {
+        var sequencer = MakeSequencer();
+        var state = new AnimatedEntityState(sequencer);
+
+        Assert.Same(sequencer, state.Sequencer);
+    }
+
+    [Fact]
+    public void Construct_WithNullSequencer_ThrowsArgumentNull()
+    {
+        Assert.Throws<System.ArgumentNullException>(
+            () => new AnimatedEntityState(null!));
+    }
+
+    private static AnimatedEntityState MakeState() => new(MakeSequencer());
+
+    private static AnimationSequencer MakeSequencer()
+        => new AnimationSequencer(new Setup(), new MotionTable(), new NullAnimationLoader());
+
+    private sealed class NullAnimationLoader : IAnimationLoader
+    {
+        public Animation? LoadAnimation(uint id) => null;
+    }
+}