feat(render): RetailChaseCamera.ViewerCellId — the swept viewer cell (retail viewer_cell)

Update() now always sets ViewerCellId: the camera-collision sweep's swept cell when collision
is on (retail viewer_cell = sphere_path.curr_cell), else the passed player cell. This is the
robust, per-frame, graph-tracked 'which cell is the camera in?' answer that V1 roots the render
on — no AABB, no grace frames (the U.4c flap source). 176 App tests green (2 new).

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

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

@@ -477,6 +477,38 @@ public class RetailChaseCameraTests
         Assert.Equal(collided, cam.Position);
     }
 
+    [Fact]
+    public void Update_WithProbeAndFlagOn_ExposesSweptViewerCell()
+    {
+        CameraDiagnostics.CollideCamera = true;
+        var probe = new FakeProbe { ReturnEye = new Vector3(1f, 2f, 3f), ReturnCell = 0xA9B40170u };
+        var cam = new RetailChaseCamera { CollisionProbe = probe };
+
+        cam.Update(
+            playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+            isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+            cellId: 0xA9B40171u, selfEntityId: 0x5);
+
+        Assert.Equal(0xA9B40170u, cam.ViewerCellId);   // the swept cell, not the player cell
+    }
+
+    [Fact]
+    public void Update_FlagOff_ViewerCellFallsBackToPlayerCell()
+    {
+        CameraDiagnostics.CollideCamera = false;
+        try
+        {
+            var cam = new RetailChaseCamera { CollisionProbe = new FakeProbe { ReturnCell = 0xDEADu } };
+            cam.Update(
+                playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+                isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+                cellId: 0xA9B40171u, selfEntityId: 0x5);
+
+            Assert.Equal(0xA9B40171u, cam.ViewerCellId);   // collision off → the passed player cell
+        }
+        finally { CameraDiagnostics.CollideCamera = true; }
+    }
+
     [Fact]
     public void Update_FlagOff_DoesNotConsultProbe()
     {