phase(N.4) Adjustment 6: add PartOverrides + HiddenPartsMask to WorldEntity

Resolves Adjustment 4 (Option A): WorldEntity now carries the server-
sent AnimPartChange data as PartOverrides and a HiddenPartsMask bitmask.
EntitySpawnAdapter.OnCreate populates AnimatedEntityState from these
fields at spawn time. GameWindow's CreateObject handler converts the
network-layer AnimPartChange records into lightweight PartOverride
structs.

This unblocks Task 22: the WbDrawDispatcher can now resolve per-part
GfxObj overrides and hidden-part suppression from entity state.

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

src/AcDream.Core/World/WorldEntity.cs

@@ -55,4 +55,27 @@ public sealed class WorldEntity
     /// visible trunk, producing "partial passthrough" bugs.
     /// </summary>
     public float Scale { get; init; } = 1.0f;
+
+    /// <summary>
+    /// Server-sent part-swap overrides from <c>AnimPartChange</c>. Each entry
+    /// replaces a Setup part's GfxObj with an alternate model (clothing, weapons,
+    /// helmets). Carried on the entity so <c>EntitySpawnAdapter</c> can populate
+    /// <c>AnimatedEntityState</c>'s override map at spawn time. Empty for atlas-
+    /// tier entities.
+    /// </summary>
+    public IReadOnlyList<PartOverride> PartOverrides { get; init; } = Array.Empty<PartOverride>();
+
+    /// <summary>
+    /// Bitmask of hidden Setup parts. Bit <c>i</c> set hides part <c>i</c> at
+    /// draw time. Sourced from the server's <c>CreateObject</c> record when
+    /// present. Zero (no parts hidden) is the default.
+    /// </summary>
+    public ulong HiddenPartsMask { get; init; }
 }
+
+/// <summary>
+/// Lightweight value type for a server-sent <c>AnimPartChange</c> (part index
+/// → replacement GfxObj id). Decouples <c>WorldEntity</c> (Core) from the
+/// network-layer <c>CreateObject.AnimPartChange</c> type.
+/// </summary>
+public readonly record struct PartOverride(byte PartIndex, uint GfxObjId);