feat(render): Phase A8 — IndoorCellStencilPipeline + PortalMeshBuilder

The pipeline class owns the portal_stencil shader + a dynamic VBO/VAO
for per-frame portal triangle uploads. MarkAndPunch runs WB's two-step
stencil setup (mark portals = 1, then write gl_FragDepth=1.0 into
stencil=1 regions). EnableOutdoorPass switches to read-only stencil
for the subsequent terrain + outdoor-entity passes.

PortalMeshBuilder.BuildTriangles is the pure-math triangle-fan
extractor — unit-testable without a GL context. Only exit portals
(OtherCellId == 0xFFFF) are emitted; inner portals are skipped to
prevent outdoor geometry from bleeding into adjacent rooms.

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

tests/AcDream.App.Tests/Rendering/IndoorCellStencilPipelineTests.cs

@@ -0,0 +1,151 @@
+// Phase A8 — portal mesh triangle-fan generation tests.
+//
+// Pure-math coverage of PortalMeshBuilder.BuildTriangles — the part of
+// IndoorCellStencilPipeline that converts a list of LoadedCell with
+// PortalPolygons + WorldTransform into a flat Vector3[] of triangles
+// in world space. The GL/upload portion is exercised at runtime only.
+
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class IndoorCellStencilPipelineTests
+{
+    [Fact]
+    public void BuildTriangles_NoCells_ReturnsEmpty()
+    {
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell>());
+        Assert.Empty(verts);
+    }
+
+    [Fact]
+    public void BuildTriangles_SkipsInnerPortals()
+    {
+        // Two portals on one cell: one exit (OtherCellId=0xFFFF, should be
+        // included), one inner (OtherCellId=0x0102, should be skipped).
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new()
+            {
+                new CellPortalInfo(0xFFFF, 100, 0),  // exit — included
+                new CellPortalInfo(0x0102, 101, 0),  // inner — skipped
+            },
+            ClipPlanes = new() { default, default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(1, 1, 0),
+                },
+                new[]
+                {
+                    new Vector3(10, 0, 0),
+                    new Vector3(11, 0, 0),
+                    new Vector3(11, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        // Only the exit polygon (3 verts → 1 triangle → 3 vertices).
+        Assert.Equal(3, verts.Length);
+        Assert.Equal(new Vector3(0, 0, 0), verts[0]);
+        Assert.Equal(new Vector3(1, 0, 0), verts[1]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[2]);
+    }
+
+    [Fact]
+    public void BuildTriangles_TriangulatesAsFan()
+    {
+        // 4-vertex quad → fan = 2 triangles → 6 vertices.
+        // Quad: (0,0,0), (1,0,0), (1,1,0), (0,1,0).
+        // Fan from vertex 0: (0,1,2), (0,2,3).
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new() { new CellPortalInfo(0xFFFF, 100, 0) },
+            ClipPlanes = new() { default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(1, 1, 0),
+                    new Vector3(0, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        Assert.Equal(6, verts.Length);
+        // Triangle 1: (0,1,2)
+        Assert.Equal(new Vector3(0, 0, 0), verts[0]);
+        Assert.Equal(new Vector3(1, 0, 0), verts[1]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[2]);
+        // Triangle 2: (0,2,3)
+        Assert.Equal(new Vector3(0, 0, 0), verts[3]);
+        Assert.Equal(new Vector3(1, 1, 0), verts[4]);
+        Assert.Equal(new Vector3(0, 1, 0), verts[5]);
+    }
+
+    [Fact]
+    public void BuildTriangles_AppliesWorldTransform()
+    {
+        // Identity cell-local triangle, translated by WorldTransform.
+        var translate = Matrix4x4.CreateTranslation(new Vector3(100, 200, 300));
+        var cell = new LoadedCell
+        {
+            WorldTransform = translate,
+            Portals = new() { new CellPortalInfo(0xFFFF, 100, 0) },
+            ClipPlanes = new() { default },
+            PortalPolygons = new()
+            {
+                new[]
+                {
+                    new Vector3(0, 0, 0),
+                    new Vector3(1, 0, 0),
+                    new Vector3(0, 1, 0),
+                },
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+
+        Assert.Equal(3, verts.Length);
+        Assert.Equal(new Vector3(100, 200, 300), verts[0]);
+        Assert.Equal(new Vector3(101, 200, 300), verts[1]);
+        Assert.Equal(new Vector3(100, 201, 300), verts[2]);
+    }
+
+    [Fact]
+    public void BuildTriangles_SkipsEmptyOrDegeneratePolygons()
+    {
+        var cell = new LoadedCell
+        {
+            WorldTransform = Matrix4x4.Identity,
+            Portals = new()
+            {
+                new CellPortalInfo(0xFFFF, 100, 0),
+                new CellPortalInfo(0xFFFF, 101, 0),
+            },
+            ClipPlanes = new() { default, default },
+            PortalPolygons = new()
+            {
+                System.Array.Empty<Vector3>(),                 // empty
+                new[] { new Vector3(0, 0, 0), new Vector3(1, 0, 0) },  // degenerate (2 verts)
+            },
+        };
+
+        var verts = PortalMeshBuilder.BuildTriangles(new List<LoadedCell> { cell });
+        Assert.Empty(verts);
+    }
+}