refactor(world): extract live projection mechanics

Move appearance rebinding, collision construction, default-pose resolution, local shadow ownership, and exact leave-world presentation into focused owners. Preserve retail parent ordering with staged validation, committed recovery, recursive attached-subtree withdrawal, and retryable exact teardown across parent, pickup, position, unwield, and pose-loss edges.

Co-Authored-By: OpenAI Codex <codex@openai.com>

src/AcDream.Core/Physics/ShadowObjectRegistry.cs

@@ -216,6 +216,92 @@ public sealed class ShadowObjectRegistry
             CollisionType: ShadowCollisionType.BSP, CylHeight: 0f, Scale: 1f);
     }
 
+    /// <summary>
+    /// Replaces an existing live PartArray collision payload in its current
+    /// shadow-cell membership. Retail <c>CPartArray::SetPart</c> changes the
+    /// part read by later collision tests; it does not recalculate cross-cells.
+    /// A suspended owner updates only its retained payload. A newly shaped
+    /// owner may perform its first flood, then immediately suspend when its
+    /// canonical projection is Hidden, attached, or cell-less.
+    /// </summary>
+    public void ReplaceMultiPartPayload(
+        uint entityId,
+        Vector3 entityWorldPos,
+        Quaternion entityWorldRot,
+        System.Collections.Generic.IReadOnlyList<ShadowShape> shapes,
+        uint state,
+        EntityCollisionFlags flags,
+        float worldOffsetX,
+        float worldOffsetY,
+        uint landblockId,
+        uint seedCellId = 0u,
+        bool isStatic = false,
+        bool suspendIfNew = false)
+    {
+        if (!_entityReg.TryGetValue(entityId, out RegistrationRecord? prior)
+            || !prior.IsMultiPart)
+        {
+            if (shapes.Count == 0)
+                return;
+            RegisterMultiPart(
+                entityId,
+                entityWorldPos,
+                entityWorldRot,
+                shapes,
+                state,
+                flags,
+                worldOffsetX,
+                worldOffsetY,
+                landblockId,
+                seedCellId,
+                isStatic);
+            if (suspendIfNew)
+                Suspend(entityId);
+            return;
+        }
+
+        bool suspended = _suspendedEntities.Contains(entityId);
+        _entityShapes[entityId] = shapes;
+        _entityReg[entityId] = prior with
+        {
+            EntityWorldPos = entityWorldPos,
+            EntityWorldRot = entityWorldRot,
+            State = state,
+            Flags = flags,
+        };
+
+        if (suspended || !_entityToCells.TryGetValue(entityId, out List<uint>? cells))
+            return;
+
+        foreach (uint cellId in cells)
+        {
+            if (_cells.TryGetValue(cellId, out List<ShadowEntry>? entries))
+                entries.RemoveAll(entry => entry.EntityId == entityId);
+        }
+
+        foreach (ShadowShape shape in shapes)
+        {
+            Vector3 partWorldPos = entityWorldPos
+                + Vector3.Transform(shape.LocalPosition, entityWorldRot);
+            Quaternion partWorldRot = entityWorldRot * shape.LocalRotation;
+            var entry = new ShadowEntry(
+                EntityId: entityId,
+                GfxObjId: shape.GfxObjId,
+                Position: partWorldPos,
+                Rotation: partWorldRot,
+                Radius: shape.Radius,
+                CollisionType: shape.CollisionType,
+                CylHeight: shape.CylHeight,
+                Scale: shape.Scale,
+                State: state,
+                Flags: flags,
+                LocalPosition: shape.LocalPosition,
+                LocalRotation: shape.LocalRotation);
+            foreach (uint cellId in cells)
+                AddEntryToCell(entry, cellId);
+        }
+    }
+
     /// <summary>
     /// Retail flood-sphere rule (CylSphere overload, Ghidra 0x0052b9f0):
     /// when the object has cylinder shapes, each contributes one sphere at