feat(vfx #C.1.5b): SetupPartTransforms helper for per-part anchor transforms

Computes Matrix4x4 per Setup part by walking PlacementFrames[Resting] →
[Default] → first-available, matching SetupMesh.Flatten's priority.
Foundation for #56 fix: ParticleHookSink will use these to apply each
CreateParticleHook's PartIndex-relative offset to the right mesh part.

4 xUnit tests cover Resting-over-Default preference, Default fallback,
empty-PlacementFrames returns empty, DefaultScale application.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Meshing/SetupPartTransformsTests.cs

@@ -0,0 +1,118 @@
+using System.Numerics;
+using AcDream.Core.Meshing;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Tests.Meshing;
+
+public class SetupPartTransformsTests
+{
+    [Fact]
+    public void Compute_PrefersRestingPlacement_OverDefault()
+    {
+        // Resting lifts part 1 by +Z=1; Default has zero lift on every part.
+        // Compute must pick Resting (matches SetupMesh.Flatten priority).
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u, 0x01000101u },
+            DefaultScale = { Vector3.One, Vector3.One },
+            PlacementFrames =
+            {
+                [Placement.Resting] = new AnimationFrame(2)
+                {
+                    Frames =
+                    {
+                        new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity },
+                        new Frame { Origin = new Vector3(0, 0, 1f), Orientation = Quaternion.Identity },
+                    },
+                },
+                [Placement.Default] = new AnimationFrame(2)
+                {
+                    Frames =
+                    {
+                        new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity },
+                        new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity },
+                    },
+                },
+            },
+        };
+
+        var transforms = SetupPartTransforms.Compute(setup);
+
+        Assert.Equal(2, transforms.Count);
+        var probe = Vector3.Transform(Vector3.Zero, transforms[1]);
+        Assert.Equal(new Vector3(0, 0, 1f), probe);
+    }
+
+    [Fact]
+    public void Compute_FallsBackToDefault_WhenRestingMissing()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u },
+            DefaultScale = { Vector3.One },
+            PlacementFrames =
+            {
+                [Placement.Default] = new AnimationFrame(1)
+                {
+                    Frames =
+                    {
+                        new Frame { Origin = new Vector3(2f, 0, 0), Orientation = Quaternion.Identity },
+                    },
+                },
+            },
+        };
+
+        var transforms = SetupPartTransforms.Compute(setup);
+
+        Assert.Single(transforms);
+        var probe = Vector3.Transform(Vector3.Zero, transforms[0]);
+        Assert.Equal(new Vector3(2f, 0, 0), probe);
+    }
+
+    [Fact]
+    public void Compute_ReturnsEmpty_WhenNoPlacementFrames()
+    {
+        // Setup with parts but no PlacementFrames — caller's
+        // ParticleHookSink falls back to Identity per part (pre-C.1.5b
+        // behavior). Returning empty signals "no per-part data available".
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u, 0x01000101u },
+        };
+
+        var transforms = SetupPartTransforms.Compute(setup);
+
+        Assert.Empty(transforms);
+    }
+
+    [Fact]
+    public void Compute_AppliesDefaultScale_WhenPresent()
+    {
+        // DefaultScale = (2,2,2) on part 0. An input (1,1,1) should
+        // come out (2,2,2) after the part transform — confirms the
+        // CreateScale factor is present in the matrix.
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u },
+            DefaultScale = { new Vector3(2f, 2f, 2f) },
+            PlacementFrames =
+            {
+                [Placement.Resting] = new AnimationFrame(1)
+                {
+                    Frames =
+                    {
+                        new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity },
+                    },
+                },
+            },
+        };
+
+        var transforms = SetupPartTransforms.Compute(setup);
+
+        Assert.Single(transforms);
+        var probe = Vector3.Transform(new Vector3(1f, 1f, 1f), transforms[0]);
+        Assert.Equal(new Vector3(2f, 2f, 2f), probe);
+    }
+}