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>

src/AcDream.App/Rendering/ICameraCollisionProbe.cs

@@ -2,9 +2,17 @@ using System.Numerics;
 
 namespace AcDream.App.Rendering;
 
+/// <summary>
+/// Result of a camera spring-arm sweep: the collided eye position AND the cell the swept
+/// viewer-sphere ended in (retail <c>viewer_cell = sphere_path.curr_cell</c>, update_viewer
+/// pc:92871). The cell is graph-tracked by the transition — no AABB, no grace frames — so it
+/// is the robust per-frame "which cell is the camera in?" answer that roots the render.
+/// </summary>
+public readonly record struct CameraSweepResult(Vector3 Eye, uint ViewerCellId);
+
 /// <summary>
 /// Sweeps a small sphere from the camera pivot (player head) toward the
-/// desired eye and returns the stopped (non-penetrating) eye. The seam that
+/// desired eye and returns the stopped (non-penetrating) eye + its cell. The seam that
 /// lets <see cref="RetailChaseCamera"/> collide its eye without depending on
 /// the physics engine directly (and stay unit-testable with a fake).
 /// </summary>
@@ -13,8 +21,9 @@ public interface ICameraCollisionProbe
     /// <summary>
     /// Roll a collision sphere from <paramref name="pivot"/> to
     /// <paramref name="desiredEye"/>; return the position it reaches without
-    /// penetrating geometry. Returns <paramref name="desiredEye"/> unchanged
+    /// penetrating geometry AND the cell it ended in. Returns
+    /// <paramref name="desiredEye"/> + <paramref name="cellId"/> unchanged
     /// when nothing blocks the path or when <paramref name="cellId"/> is 0.
     /// </summary>
-    Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId);
+    CameraSweepResult SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId);
 }