diag(render): Phase U.4c — flap probe logs projected NDC coords + clip result

The [flap] line now reports, per root portal, the actual projected NDC vertices and
the Intersect-against-FullScreen result count (clip=N), so a portal that PROJECTS
(proj>=3) but still fails to ADD its neighbour (vis stays low) shows WHY: clip=0 with
ndc inside [-1,1] = winding/self-intersection degeneracy; clip=0 with ndc outside
[-1,1] = genuinely off-screen; the ndc coords expose a near-plane bowtie. Pins the
exact clip-region failure before the root-cause fix.

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

src/AcDream.App/Rendering/PortalVisibilityBuilder.cs

@@ -238,6 +238,10 @@ public static class PortalVisibilityBuilder
         return frame;
     }
 
+    // The NDC [-1,1] viewport quad (CCW), reused by the flap probe's clip recompute.
+    private static readonly Vector2[] FullScreenQuad =
+        { new Vector2(-1f, -1f), new Vector2(1f, -1f), new Vector2(1f, 1f), new Vector2(-1f, 1f) };
+
     // Phase U.4c flap probe. One [flap] line per Build: the root cell's per-portal
     // signed distance D (eye→portal plane), traverse/cull decision, and NDC projection
     // vertex count, plus the frame's OutsideView polygon count + visible-cell count.
@@ -266,17 +270,34 @@ public static class PortalVisibilityBuilder
                 d = Vector3.Dot(pl.Normal, localEye) + pl.D;
                 side = CameraOnInteriorSide(cameraCell, i, cameraPos);
             }
-            int projN = -1;
+            // Replicate the walk's project → EnsureCcw → Intersect(FullScreen) exactly, so a
+            // portal that PROJECTS (proj>=3) but still fails to ADD its neighbour shows WHY:
+            // clip=0 with ndc inside [-1,1] ⇒ winding/self-intersection degeneracy; clip=0 with
+            // ndc outside [-1,1] ⇒ genuinely off-screen. The ndc coords expose a near-plane bowtie.
+            int projN = -1, clipN = -1;
+            string ndcText = "";
             if (i < cameraCell.PortalPolygons.Count)
             {
                 var poly = cameraCell.PortalPolygons[i];
                 if (poly != null && poly.Length >= 3)
-                    projN = PortalProjection.ProjectToNdc(poly, cameraCell.WorldTransform, viewProj).Length;
+                {
+                    var ndc = PortalProjection.ProjectToNdc(poly, cameraCell.WorldTransform, viewProj);
+                    projN = ndc.Length;
+                    if (ndc.Length >= 3)
+                    {
+                        EnsureCcw(ndc);
+                        clipN = ScreenPolygonClip.Intersect(ndc, FullScreenQuad).Length;
+                        var ns = new System.Text.StringBuilder(48);
+                        foreach (var v in ndc) ns.Append('(').Append(v.X.ToString("F1")).Append(',').Append(v.Y.ToString("F1")).Append(')');
+                        ndcText = ns.ToString();
+                    }
+                }
             }
             sb.Append(" | p").Append(i).Append("->0x").Append(portal.OtherCellId.ToString("X4"));
             sb.Append(" D=").Append(float.IsNaN(d) ? "na" : d.ToString("F2"));
             sb.Append(side ? " TRV" : " CULL");
-            sb.Append(" proj=").Append(projN);
+            sb.Append(" proj=").Append(projN).Append(" clip=").Append(clipN);
+            if (ndcText.Length > 0) sb.Append(" ndc=").Append(ndcText);
         }
         sb.Append(" || outPolys=").Append(frame.OutsideView.Polygons.Count);
         sb.Append(" vis=").Append(frame.OrderedVisibleCells.Count);