test(picker): Cluster A #86 — screen-rect cell-occlusion tests

Phase B's WorldPicker change added cellOccluder to both Pick overloads,
but the integration test suite only covered the legacy ray-sphere
overload. The production code path (GameWindow.PickAndStoreSelection)
uses the screen-rect overload, and its clip.W depth-conversion math
had no direct test. Adds two integration tests mirroring the existing
ray-sphere variants:

- Pick_ScreenRect_EntityBehindWall_OccludedByCellBsp — entity dead-
  ahead, wall between, with cellOccluder → null.
- Pick_ScreenRect_NoWall_HitsEntity — same scene, null occluder → hit.

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

tests/AcDream.Core.Tests/Selection/WorldPickerCellOcclusionTests.cs

@@ -47,6 +47,114 @@ public class WorldPickerCellOcclusionTests
         MeshRefs = System.Array.Empty<MeshRef>(),
     };
 
+    /// <summary>
+    /// Builds a quad wall at Z=-10 in front of the camera (identity view,
+    /// camera looking down -Z). The wall spans X=-5..5, Y=-5..5 at Z=-10 —
+    /// large enough to cover the center-pixel ray. An entity at Z=-20 sits
+    /// behind it.
+    ///
+    /// Wall normal direction doesn't affect Möller-Trumbore (the occluder
+    /// is two-sided), but the Plane is stored for completeness. For a plane
+    /// at z=-10 with outward normal (0,0,+1): (0,0,1)·(x,y,-10) + D = 0
+    /// → D = 10.
+    /// </summary>
+    private static CellPhysics MakeWallAtZNeg10()
+    {
+        var verts = new[]
+        {
+            new Vector3(-5, -5, -10),
+            new Vector3( 5, -5, -10),
+            new Vector3( 5,  5, -10),
+            new Vector3(-5,  5, -10),
+        };
+        var poly = new ResolvedPolygon
+        {
+            Vertices  = verts,
+            Plane     = new System.Numerics.Plane(new Vector3(0, 0, 1), 10f),
+            NumPoints = 4,
+            SidesType = CullMode.None,
+        };
+        return new CellPhysics
+        {
+            BSP = null,
+            Resolved = new() { [0] = poly },
+            WorldTransform = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+        };
+    }
+
+    // ──────────────────────────────────────────────
+    // Screen-rect overload + cell-BSP occlusion
+    // ──────────────────────────────────────────────
+
+    /// <summary>
+    /// Production path exercised by GameWindow.PickAndStoreSelection.
+    /// Camera at origin looking down -Z (identity view). Entity at Z=-20
+    /// projects to the center of the viewport. A wall at Z=-10 sits between
+    /// camera and entity; with cellOccluder wired up the entity must be
+    /// occluded → null result.
+    ///
+    /// This test specifically covers the clip.W depth-conversion math in
+    /// WorldPicker.Pick's screen-rect overload (issue #86).
+    /// </summary>
+    [Fact]
+    public void Pick_ScreenRect_EntityBehindWall_OccludedByCellBsp()
+    {
+        // Use the same camera convention as WorldPickerRectOverloadTests.StdCam():
+        // identity view, 90-degree FoV, 800×600 viewport. Center pixel = (400,300).
+        var view     = Matrix4x4.Identity;
+        var proj     = Matrix4x4.CreatePerspectiveFieldOfView(
+                          MathF.PI * 0.5f, 800f / 600f, 0.1f, 100f);
+        var viewport = new Vector2(800f, 600f);
+
+        var wall   = MakeWallAtZNeg10();
+        var entity = MakeEntity(0xABCDu, new Vector3(0, 0, -20));
+
+        // Entity is dead-ahead: center of viewport.
+        var result = WorldPicker.Pick(
+            mouseX: 400f, mouseY: 300f,
+            view, proj, viewport,
+            candidates: new[] { entity },
+            skipServerGuid: 0u,
+            sphereForEntity: e => ((Vector3, float)?)(e.Position, 1.0f),
+            inflatePixels: 8f,
+            cellOccluder: (origin, direction) =>
+                CellBspRayOccluder.NearestWallT(origin, direction, new[] { wall }));
+
+        Assert.Null(result);
+    }
+
+    /// <summary>
+    /// Same camera and entity as Pick_ScreenRect_EntityBehindWall_OccludedByCellBsp,
+    /// but with a null cellOccluder. Verifies that the no-occluder path still
+    /// resolves the entity to a hit (the new parameter is a pure no-op when null).
+    /// </summary>
+    [Fact]
+    public void Pick_ScreenRect_NoWall_HitsEntity()
+    {
+        var view     = Matrix4x4.Identity;
+        var proj     = Matrix4x4.CreatePerspectiveFieldOfView(
+                          MathF.PI * 0.5f, 800f / 600f, 0.1f, 100f);
+        var viewport = new Vector2(800f, 600f);
+
+        var entity = MakeEntity(0xABCDu, new Vector3(0, 0, -20));
+
+        var result = WorldPicker.Pick(
+            mouseX: 400f, mouseY: 300f,
+            view, proj, viewport,
+            candidates: new[] { entity },
+            skipServerGuid: 0u,
+            sphereForEntity: e => ((Vector3, float)?)(e.Position, 1.0f),
+            inflatePixels: 8f,
+            cellOccluder: null);
+
+        Assert.Equal(0xABCDu, result);
+    }
+
+    // ──────────────────────────────────────────────
+    // Ray-sphere overload (legacy path)
+    // ──────────────────────────────────────────────
+
     [Fact]
     public void Pick_RaySphere_EntityBehindWall_OccludedByCellBsp()
     {