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>

src/AcDream.Core/Meshing/SetupPartTransforms.cs

@@ -0,0 +1,76 @@
+using System.Collections.Generic;
+using System.Numerics;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Meshing;
+
+/// <summary>
+/// Compute the per-part static transforms for a Setup using its
+/// PlacementFrames. For each part <c>i</c>, the returned matrix takes a
+/// point in part-local space and yields a point in setup-local space at
+/// the Setup's resting pose.
+///
+/// <para>
+/// Mirrors <see cref="SetupMesh.Flatten"/>'s pose-source priority —
+/// <c>PlacementFrames[Resting]</c> → <c>[Default]</c> → first available
+/// — so that a particle anchor at part <c>i</c> matches the part's
+/// visible rest position. If renderer and resolver ever drift on this
+/// priority, particles will visibly drift relative to their parent
+/// mesh; keep them in lockstep.
+/// </para>
+///
+/// <para>
+/// Returns an empty list when the Setup has no PlacementFrames. The
+/// caller (e.g. <c>ParticleHookSink.SpawnFromHook</c>) should then fall
+/// back to <see cref="Matrix4x4.Identity"/> per part, which is the
+/// pre-C.1.5b behavior.
+/// </para>
+///
+/// <para>
+/// For animated entities, per-part transforms vary per animation frame
+/// and live in <c>AnimatedEntityState</c>; a future "animated
+/// DefaultScript" path would publish those each tick via the same
+/// <c>SetEntityPartTransforms</c> seam. Out of scope here.
+/// </para>
+/// </summary>
+public static class SetupPartTransforms
+{
+    public static IReadOnlyList<Matrix4x4> Compute(Setup setup)
+    {
+        AnimationFrame? source = null;
+        if (setup.PlacementFrames.TryGetValue(Placement.Resting, out var resting))
+        {
+            source = resting;
+        }
+        else if (setup.PlacementFrames.TryGetValue(Placement.Default, out var def))
+        {
+            source = def;
+        }
+        else
+        {
+            foreach (var kvp in setup.PlacementFrames)
+            {
+                source = kvp.Value;
+                break;
+            }
+        }
+
+        if (source is null) return System.Array.Empty<Matrix4x4>();
+
+        int partCount = setup.Parts.Count;
+        var result = new Matrix4x4[partCount];
+        for (int i = 0; i < partCount; i++)
+        {
+            Frame frame = i < source.Frames.Count
+                ? source.Frames[i]
+                : new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity };
+            Vector3 scale = i < setup.DefaultScale.Count ? setup.DefaultScale[i] : Vector3.One;
+            result[i] = Matrix4x4.CreateScale(scale)
+                      * Matrix4x4.CreateFromQuaternion(frame.Orientation)
+                      * Matrix4x4.CreateTranslation(frame.Origin);
+        }
+        return result;
+    }
+}