test(N.4): conformance tests for mesh extraction + setup flatten

Mesh extraction (4 tests): quad output, double-sided via Stippling.Both,
double-sided via SidesType=Clockwise (AC's NoNeg-clear convention),
NoPos-only emission. Pins GfxObjMesh.Build's behavior.

Setup flatten (5 tests): identity (no frames), Default frame, Resting
beats Default, motion override beats Resting, DefaultScale per part.
Pins SetupMesh.Flatten's placement-frame fallback chain.

These run BEFORE substitution per N.1/N.3 pattern — they prove
equivalence, not test the substitution.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Rendering/Wb/SetupFlattenConformanceTests.cs

@@ -0,0 +1,105 @@
+using System.Numerics;
+using AcDream.Core.Meshing;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+/// <summary>
+/// Conformance: our <see cref="SetupMesh.Flatten"/> must produce the same
+/// (GfxObjId, Matrix4x4) sequence as WB's setup-parts walk for representative
+/// Setups. Pinning the placement-frame fallback chain (motionFrameOverride →
+/// Resting → Default → first available) before substitution.
+/// </summary>
+public sealed class SetupFlattenConformanceTests
+{
+    [Fact]
+    public void Flatten_NoFrames_FallsBackToIdentity()
+    {
+        var setup = new Setup { Parts = { 0x01000001u } };
+        // PlacementFrames deliberately empty — no DefaultScale entry either,
+        // so scale defaults to Vector3.One and the fallback frame is
+        // (Origin=Zero, Orientation=Identity) → Identity matrix.
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Single(refs);
+        Assert.Equal(0x01000001u, refs[0].GfxObjId);
+        Assert.Equal(Matrix4x4.Identity, refs[0].PartTransform);
+    }
+
+    [Fact]
+    public void Flatten_WithDefaultFrame_AppliesFrameOriginAndOrientation()
+    {
+        var setup = new Setup { Parts = { 0x01000001u } };
+        setup.PlacementFrames[Placement.Default] = new AnimationFrame(1)
+        {
+            Frames =
+            {
+                new Frame
+                {
+                    Origin = new Vector3(10, 20, 30),
+                    Orientation = Quaternion.Identity,
+                },
+            },
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Equal(new Vector3(10, 20, 30), refs[0].PartTransform.Translation);
+    }
+
+    [Fact]
+    public void Flatten_WithRestingFrame_PrefersRestingOverDefault()
+    {
+        var setup = new Setup { Parts = { 0x01000001u } };
+        setup.PlacementFrames[Placement.Default] = new AnimationFrame(1)
+        {
+            Frames = { new Frame { Origin = new Vector3(10, 20, 30), Orientation = Quaternion.Identity } },
+        };
+        setup.PlacementFrames[Placement.Resting] = new AnimationFrame(1)
+        {
+            Frames = { new Frame { Origin = new Vector3(99, 99, 99), Orientation = Quaternion.Identity } },
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Equal(new Vector3(99, 99, 99), refs[0].PartTransform.Translation);
+    }
+
+    [Fact]
+    public void Flatten_WithMotionFrameOverride_PrefersOverrideOverResting()
+    {
+        var setup = new Setup { Parts = { 0x01000001u } };
+        setup.PlacementFrames[Placement.Resting] = new AnimationFrame(1)
+        {
+            Frames = { new Frame { Origin = new Vector3(99, 99, 99), Orientation = Quaternion.Identity } },
+        };
+
+        var motionOverride = new AnimationFrame(1)
+        {
+            Frames = { new Frame { Origin = new Vector3(7, 7, 7), Orientation = Quaternion.Identity } },
+        };
+
+        var refs = SetupMesh.Flatten(setup, motionFrameOverride: motionOverride);
+
+        Assert.Equal(new Vector3(7, 7, 7), refs[0].PartTransform.Translation);
+    }
+
+    [Fact]
+    public void Flatten_DefaultScalePerPart_AppliedToTransform()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000001u, 0x01000002u },
+            DefaultScale = { new Vector3(2, 2, 2), new Vector3(3, 3, 3) },
+        };
+        // No placement frames — fallback frame is identity pose; scale still applies.
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Equal(2f, refs[0].PartTransform.M11);
+        Assert.Equal(3f, refs[1].PartTransform.M11);
+    }
+}