fix(app): terrain cull exempt + yaw preservation on Tab toggle

1. Terrain culling: TerrainRenderer.Draw now accepts neverCullLandblockId,
   matching StaticMeshRenderer. Both renderers skip frustum-culling the
   player's current landblock. Previously only entities were exempt but
   the terrain under the player still disappeared when looking away.

2. Yaw drift on Tab toggle: render loop stores rotation as Yaw - PI/2
   (AC model facing offset), but yaw extraction on re-entering player
   mode didn't compensate. Each Tab cycle rotated the player 90 degrees.
   Now adds PI/2 back when extracting from the quaternion.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -224,13 +224,15 @@ public sealed class GameWindow : IDisposable
                                 playerEntity.Position, pinitCellId & 0xFFFFu,
                                 System.Numerics.Vector3.Zero, 100f);  // huge step height for initial snap
                             _playerController.SetPosition(initResult.Position, initResult.CellId);
-                            // Derive initial yaw from the entity's server-sent rotation
+                            // Derive initial yaw from the entity's rotation.
-                            // rather than hardcoding. Extract yaw from the quaternion.
+                            // The render loop stores rotation as Yaw - PI/2 (to
+                            // compensate for AC models facing +Y at identity), so
+                            // we add PI/2 back when extracting to get the real yaw.
                             var q = playerEntity.Rotation;
-                            float yaw = MathF.Atan2(
+                            float rawYaw = MathF.Atan2(
                                 2f * (q.W * q.Z + q.X * q.Y),
                                 1f - 2f * (q.Y * q.Y + q.Z * q.Z));
-                            _playerController.Yaw = yaw;
+                            _playerController.Yaw = rawYaw + MathF.PI / 2f;
 
                             _chaseCamera = new AcDream.App.Rendering.ChaseCamera
                             {
@@ -1756,7 +1758,7 @@ public sealed class GameWindow : IDisposable
         {
             var camera = _cameraController.Active;
             var frustum = AcDream.App.Rendering.FrustumPlanes.FromViewProjection(camera.View * camera.Projection);
-            _terrain?.Draw(camera, frustum);
+
             // Never cull the landblock the player is currently on.
             uint? playerLb = null;
             if (_playerMode && _playerController is not null)
@@ -1766,6 +1768,7 @@ public sealed class GameWindow : IDisposable
                 int ply = _liveCenterY + (int)System.Math.Floor(pp.Y / 192f);
                 playerLb = (uint)((plx << 24) | (ply << 16) | 0xFFFF);
             }
+            _terrain?.Draw(camera, frustum, neverCullLandblockId: playerLb);
             _staticMesh?.Draw(camera, _worldState.LandblockEntries, frustum,
                 neverCullLandblockId: playerLb);
 

src/AcDream.App/Rendering/TerrainRenderer.cs

@@ -116,7 +116,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
         _landblocks.Remove(landblockId);
     }
 
-    public void Draw(ICamera camera, FrustumPlanes? frustum = null)
+    public void Draw(ICamera camera, FrustumPlanes? frustum = null, uint? neverCullLandblockId = null)
     {
         _shader.Use();
         _shader.SetMatrix4("uView", camera.View);
@@ -135,7 +135,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
 
         foreach (var lb in _landblocks.Values)
         {
-            if (frustum is not null)
+            if (frustum is not null && lb.LandblockId != neverCullLandblockId)
             {
                 var aabbMin = new Vector3(lb.WorldOrigin.X, lb.WorldOrigin.Y, lb.MinZ);
                 var aabbMax = new Vector3(lb.WorldOrigin.X + 192f, lb.WorldOrigin.Y + 192f, lb.MaxZ);