fix(core+app): Phase B.3 — Setup.StepUpHeight + scenery road exclusion

Four targeted fixes for user-reported movement/visual bugs:

1. Player entity disappearing: GpuWorldState now supports persistent
   entities (MarkPersistent/DrainRescued). The player character survives
   landblock unloads and gets re-injected into the streaming window at
   the current center landblock.

2. Feet sinking into terrain: +0.15 Z bias in PlayerMovementController
   keeps the character model above terrain z-fighting edge cases.

3. Camera after portal teleport: ChaseCamera.Update now called
   immediately after teleport snap so the camera recenters on the new
   position instead of lingering at the pre-teleport location.

4. Scenery on roads: SceneryGenerator now checks road status at the
   final displaced position (not just the origin vertex), catching
   objects that drift from non-road vertices onto road cells.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -198,7 +198,8 @@ public sealed class PlayerMovementController
             }
         }
 
-        Position = new Vector3(result.Position.X, result.Position.Y, newZ);
+        // Small upward bias prevents feet from z-fighting with terrain surface.
+        Position = new Vector3(result.Position.X, result.Position.Y, newZ + 0.15f);
         CellId = result.CellId;
 
         // 4. Determine current motion commands.