docs(render): Phase W Stage 4 — verify ceiling-sealed / terrain-viewpoint / GL-state + ParentCellId

T4.4: annotate EnvCellRenderer.RegisterCell to document that ceilings are present by
construction. PrepareCellStructMeshData iterates ALL CellStruct.Polygons (floor + walls +
ceiling) with no surface filter; retail PView::DrawCells draws the same closed-box
drawing_bsp. No ceiling filtering confirmed.

T4.2: annotate TerrainModernRenderer.Draw to document that terrain projects from the
passed-in ICamera (uView + uProjection derive from the same camera as all other
renderers). No separate landscape viewpoint exists that could desync from the eye.

T4.5, T5.1, T5.2: pure verification — no code changes (see report).

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

src/AcDream.App/Rendering/TerrainModernRenderer.cs

@@ -260,6 +260,12 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
         }
 
         // Bind shader + uniforms + atlas handles.
+        // Verified Phase W Stage 4 (T4.2): terrain projects from the camera view-proj;
+        // no separate landscape viewpoint to sync. Both uView and uProjection derive
+        // from the ICamera passed into this method — the same camera used for all other
+        // renderers in the unified pipeline. Retail's LScape::update_viewpoint
+        // pre-positions terrain to the outdoor landcell, but acdream uses the
+        // unified camera matrix everywhere, so no separate viewpoint divergence can occur.
         _shader.Use();
         _shader.SetMatrix4("uView", camera.View);
         _shader.SetMatrix4("uProjection", camera.Projection);