fix(render): clip portal projection against frustum side planes (clip-space)

ProjectToNdc clipped only the eye half-space (w>MinW, a 2026-06-03 workaround) and left the 4 frustum side planes to the 2D ScreenPolygonClip. When the eye is within a portal's near plane, small-w verts explode under the perspective divide (probe saw NDC (10.2,-67.4)); the 2D clip then collapses to empty -> OutsideView empty -> terrain Skip -> the bluish doorway void. Clip the eye plane + 4 side planes (homogeneous Sutherland-Hodgman) before the divide so NDC is bounded to the screen by construction, matching retail GetClip -> polyClipFinish (clip in clip-space before the divide; pc:432344).

Partial: NOT the full flicker fix. The dominant cause (camera boom drift + viewer-cell flip at boundaries + missing w=0 near-plane clip) is identified and deferred to the next session per the handoff. 2 RED->GREEN tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -86,6 +86,59 @@ public class PortalProjectionTests
         }
     }
 
+    [Fact]
+    public void Project_QuadStraddlingCamera_NdcStaysWithinScreen()
+    {
+        // R1 void-flap fix (2026-06-05): the eye-plane-only clip (w>MinW) let small-w verts
+        // explode under the perspective divide (~±37 NDC). Those off-screen NDC then corrupted
+        // the downstream 2D ScreenPolygonClip, which at glancing/close angles collapsed to EMPTY
+        // -> OutsideView empty -> terrain Skip -> the bluish "void" at the cottage doorway.
+        // Clipping the 4 frustum SIDE planes in clip space (homogeneous, before the divide)
+        // bounds every projected vertex to the screen [-1,1] by construction. RED before the fix.
+        var poly = new[]
+        {
+            new Vector3(-1, -1, 2), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, 2),
+        };
+        var r = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
+        Assert.True(r.Length >= 3);
+        foreach (var v in r)
+        {
+            Assert.InRange(v.X, -1.001f, 1.001f); // bounded to the screen — no off-screen explosion
+            Assert.InRange(v.Y, -1.001f, 1.001f);
+        }
+    }
+
+    [Fact]
+    public void Project_CloseDoorway_NdcStaysWithinScreen_AndCoversScreen()
+    {
+        // The probe-confirmed void frame (2026-06-05): the chase eye is ~0.28 m from the front-door
+        // EXIT portal — well inside RetailChaseCamera's 1.0 m near plane — and looking through it.
+        // The door subtends the whole screen, but the old clip produced NDC like (10.2,-67.4) and
+        // ScreenPolygonClip reduced it to clip=0 (the void). After the homogeneous side-plane clip
+        // the NDC stays on-screen AND the door still covers the viewport (non-empty), not the void.
+        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
+        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1.0f, 5000f); // RetailChaseCamera
+        var viewProj = view * proj;
+
+        // A 2 m x 2 m doorway 0.28 m in front of the eye, facing it.
+        var doorway = new[]
+        {
+            new Vector3(-1f, -1f, -0.28f), new Vector3(1f, -1f, -0.28f),
+            new Vector3(1f,  1f, -0.28f),  new Vector3(-1f, 1f, -0.28f),
+        };
+        var projected = PortalProjection.ProjectToNdc(doorway, Matrix4x4.Identity, viewProj);
+        Assert.True(projected.Length >= 3);
+        foreach (var v in projected)
+        {
+            Assert.InRange(v.X, -1.001f, 1.001f);
+            Assert.InRange(v.Y, -1.001f, 1.001f);
+        }
+
+        var viewport = CellView.FullScreen().Polygons[0].Vertices;
+        var onScreen = ScreenPolygonClip.Intersect(projected, viewport);
+        Assert.True(onScreen.Length >= 3, "a doorway the eye looks through must cover the screen, not collapse to the void");
+    }
+
     [Fact]
     public void Project_PortalEyeIsAlmostTouching_StaysVisibleOnScreen()
     {