docs(render): Phase A8 — document portal_stencil.vert pos.w omission

WB's PortalStencil.vert has a pos.w clamp for the camera-coplanar-with-
portal degenerate. We exclude it per spec (matches retail intent), but
the file should note the omission so future readers don't wonder.

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

src/AcDream.App/Rendering/Shaders/portal_stencil.vert

@@ -10,6 +10,9 @@ layout(location = 0) in vec3 aPosition;
 
 uniform mat4 uViewProjection;
 
+// Note: no pos.w clamp — coplanar-camera degenerate is accepted per spec.
+// If stencil artifacts appear when the camera straddles an exit portal plane,
+// re-introduce the clamp from WB's PortalStencil.vert.
 void main()
 {
     gl_Position = uViewProjection * vec4(aPosition, 1.0);