feat(render): R1 — InteriorEntityPartition (3-bucket per-cell entity split)

Pure helper splitting a frame's entities into live-dynamic / per-cell statics /
outdoor scenery, by the same precedence as WbDrawDispatcher.ResolveEntitySlot
(serverGuid first — live entities have no ParentCellId). Feeds the per-cell
DrawInside loop. 3 unit tests, GL-free.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/InteriorEntityPartition.cs

@@ -0,0 +1,62 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.World;
+
+namespace AcDream.App.Rendering;
+
+/// <summary>
+/// Splits a frame's landblock entities into the three draw buckets the per-cell
+/// <see cref="InteriorRenderer"/> needs, using the SAME precedence as
+/// <see cref="Wb.WbDrawDispatcher.ResolveEntitySlot"/>:
+/// <list type="number">
+/// <item><b>ServerGuid != 0</b> (player / NPCs / items / doors) ⇒ <see cref="Result.LiveDynamic"/>
+///   — drawn unclipped (depth only). These have no <c>ParentCellId</c> so they MUST be tested first.</item>
+/// <item><b>ParentCellId</b> in the visible set ⇒ <see cref="Result.ByCell"/>[cell] — per-cell, portal-clipped.</item>
+/// <item><b>ParentCellId == null</b> (outdoor scenery / building shell) ⇒ <see cref="Result.Outdoor"/>
+///   — drawn through the doorway, clipped to OutsideView.</item>
+/// </list>
+/// A static whose <c>ParentCellId</c> is NOT in <paramref name="visibleCells"/> is dropped (its cell
+/// isn't drawn this frame). Entities with no <c>MeshRefs</c> are skipped. Pure; GL-free; unit-tested.
+/// </summary>
+public static class InteriorEntityPartition
+{
+    public sealed class Result
+    {
+        public Dictionary<uint, List<WorldEntity>> ByCell { get; } = new();
+        public List<WorldEntity> Outdoor { get; } = new();
+        public List<WorldEntity> LiveDynamic { get; } = new();
+    }
+
+    public static Result Partition(
+        HashSet<uint> visibleCells,
+        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
+                     IReadOnlyList<WorldEntity> Entities,
+                     IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> landblockEntries)
+    {
+        var result = new Result();
+        foreach (var entry in landblockEntries)
+        {
+            foreach (var e in entry.Entities)
+            {
+                if (e.MeshRefs.Count == 0) continue;
+
+                if (e.ServerGuid != 0)            // live-dynamic — precedence first (no ParentCellId)
+                {
+                    result.LiveDynamic.Add(e);
+                }
+                else if (e.ParentCellId is uint cell)
+                {
+                    if (!visibleCells.Contains(cell)) continue;   // its cell isn't drawn this frame
+                    if (!result.ByCell.TryGetValue(cell, out var list))
+                        result.ByCell[cell] = list = new List<WorldEntity>();
+                    list.Add(e);
+                }
+                else                              // outdoor scenery / building shell
+                {
+                    result.Outdoor.Add(e);
+                }
+            }
+        }
+        return result;
+    }
+}