refactor(render): SweepEye returns (Eye, ViewerCellId) — surface the swept viewer cell

The camera spring-arm sweep already resolves the collided eye's cell (ResolveResult.CellId
= sp.CurCellId = retail viewer_cell = sphere_path.curr_cell, update_viewer pc:92871).
Return it from SweepEye so the render can root on the viewer cell (Phase W single-viewpoint
V1, Task 1). Pure plumbing — behavior unchanged; callers extract .Eye. 174 App tests green.

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

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

@@ -38,6 +38,7 @@ public class PhysicsCameraCollisionProbeTests
 
         var result = probe.SweepEye(pivot, eye, cellId: 0, selfEntityId: 0);
 
-        Assert.Equal(eye, result);
+        Assert.Equal(eye, result.Eye);
+        Assert.Equal(0u, result.ViewerCellId);   // cellId==0 → returned unchanged
     }
 }