feat(core): add SetupMesh.Flatten for single-level part hierarchy

src/AcDream.Core/Meshing/SetupMesh.cs

@@ -0,0 +1,45 @@
+using System.Numerics;
+using AcDream.Core.World;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Meshing;
+
+public static class SetupMesh
+{
+    /// <summary>
+    /// Flatten a Setup into a list of (GfxObjId, PartTransform) refs.
+    /// Uses the default placement frame and DefaultScale per part.
+    /// Does NOT walk ParentIndex — each part's transform is local to the setup root.
+    /// This is simplification for Phase 2; complex hierarchical rigs are Phase 3.
+    /// </summary>
+    public static IReadOnlyList<MeshRef> Flatten(Setup setup)
+    {
+        AnimationFrame? defaultAnim = null;
+        if (setup.PlacementFrames.TryGetValue(Placement.Default, out var af))
+            defaultAnim = af;
+
+        var result = new List<MeshRef>(setup.Parts.Count);
+        for (int i = 0; i < setup.Parts.Count; i++)
+        {
+            uint gfxObjId = (uint)setup.Parts[i];
+
+            Frame frame;
+            if (defaultAnim is not null && i < defaultAnim.Frames.Count)
+                frame = defaultAnim.Frames[i];
+            else
+                frame = new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity };
+
+            Vector3 scale = i < setup.DefaultScale.Count ? setup.DefaultScale[i] : Vector3.One;
+
+            var transform =
+                Matrix4x4.CreateScale(scale) *
+                Matrix4x4.CreateFromQuaternion(frame.Orientation) *
+                Matrix4x4.CreateTranslation(frame.Origin);
+
+            result.Add(new MeshRef(gfxObjId, transform));
+        }
+        return result;
+    }
+}