fix(render) Campaign FW3 visual gate: sweep-fail frames root on the PLAYER cell

Owner report: floor openings (stairs down) briefly covered by outside
terrain at camera in/out crossings - the historic #119 family
transition arm. Root cause: at a doorway graze the camera collision
sweep can transiently fail; retail total-failure fallback
set_viewer(&player->m_position, 1) (update_viewer @0x00453fad, and the
cell==0 bail @0x00453d1d) copies the WHOLE player Position - objcell_id
included - so retail roots those frames on the player interior cell.
Our fallback left ViewerCellId = 0, which the walk reads as an OUTDOOR
root while the eye is inside: full-screen terrain with the look-in
gates correctly refusing from the interior side - grass over the
stairwell for exactly the failing frames. The fallback now inherits
the pivot/player cell, matching retail byte-for-byte.

Suites: full Release build 0 warnings; hermetic 6,758/0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/RetailChaseCamera.cs

@@ -252,12 +252,23 @@ public sealed class RetailChaseCamera : ICamera
             var swept = CollisionProbe.SweepEye(pivotWorld, _soughtEye, cellId, selfEntityId, playerPosition);
             publishedEye = swept.Eye;
             ViewerCellId = swept.ViewerCellId;
-            // Total-failure fallback = retail set_viewer(player_pos, reset_sought=1)
+            // Total-failure fallback = retail set_viewer(&player->m_position,
-            // (update_viewer :92886 and the cell==0 bail :92775 — both surface here as
+            // reset_sought=1) (update_viewer @0x00453fad and the cell==0 bail
-            // ViewerCellId == 0): the sought resets to the returned position and
+            // @0x00453d1d — both surface here as ViewerCellId == 0): the
-            // re-extends from there.
+            // sought resets to the returned position and re-extends, AND the
+            // viewer INHERITS THE PLAYER'S POSITION CELL — set_viewer copies
+            // the whole Position, objcell_id included, so retail ROOTS ON THE
+            // PLAYER'S (interior) CELL for these frames. Leaving 0 here made
+            // the walk read the frame as OUTDOOR-rooted while the eye was
+            // inside a doorway graze: full-screen terrain with the look-in
+            // gates correctly refusing from the interior side — the FW3
+            // visual gate's "grass briefly covers the stairwell at the
+            // crossing" report (the historic #119 family's transition arm).
             if (swept.ViewerCellId == 0)
+            {
                 _soughtEye = swept.Eye;
+                ViewerCellId = cellId;
+            }
         }
         // Retail viewer — the base of next frame's interpolation (step 5).
         _publishedEye = publishedEye;