fix(app+core): Phase B.3 — player cull-exempt, jump height, slope Z

Three user-reported movement fixes:

1. Player disappears when facing away: StaticMeshRenderer now accepts
   an alwaysVisibleEntityId. When a culled landblock contains the
   player entity, it is still drawn. Prevents the frustum culler from
   hiding the player character when they walk far from their spawn
   landblock.

2. Jump too high: JumpImpulse reduced from 10.0 to 3.5 (placeholder;
   retail scales by Jump skill value from the server).

3. Slope Z alignment: replaced the frame-delta slope bias with a
   foot-forward sampling approach — sample terrain Z at 1 unit ahead
   in the walk direction and use max(center, foot) as the ground Z.
   Handles multi-grade slopes where the terrain rises faster than a
   single-point sample tracks.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -1746,7 +1746,8 @@ public sealed class GameWindow : IDisposable
             var camera = _cameraController.Active;
             var frustum = AcDream.App.Rendering.FrustumPlanes.FromViewProjection(camera.View * camera.Projection);
             _terrain?.Draw(camera, frustum);
-            _staticMesh?.Draw(camera, _worldState.LandblockEntries, frustum);
+            _staticMesh?.Draw(camera, _worldState.LandblockEntries, frustum,
+                alwaysVisibleEntityId: _playerMode ? _playerServerGuid : null);
 
             // Count visible vs total for the perf overlay.
             foreach (var entry in _worldState.LandblockEntries)