fix(app+core): Phase B.2 — increase step height + resolve initial Z from terrain

Two fixes for the "position never changes when walking" bug:

1. StepUpHeight was 1.0 units — too tight. The player started at
   Z=92.2 (ACE relocation from previous session) but terrain Z was
   ~94, so every movement attempt had a Z delta of 1.8 which
   exceeded the limit. Increased to 5.0 (forgiving for MVP; AC
   default for humans is ~2 from Setup.StepUpHeight).

2. Initial position now resolves through PhysicsEngine with a huge
   step height (100) to snap to the correct terrain Z regardless
   of where the server-sent Z currently is. With indoor transitions
   disabled, this always produces the outdoor terrain height.

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -44,7 +44,14 @@ public sealed class PlayerMovementController
     public float RunSpeed { get; set; } = 7f;
     public float TurnSpeed { get; set; } = 1.5f;
     public float MouseTurnSensitivity { get; set; } = 0.003f;
-    public float StepUpHeight { get; set; } = 1.0f;
+    /// <summary>
+    /// Maximum Z increase per movement step before the move is rejected.
+    /// AC's default StepUpHeight for human characters is ~2 units (from
+    /// Setup.StepUpHeight). Using 5 for the MVP to be forgiving — prevents
+    /// walking up vertical walls but allows stairs, ramps, and terrain
+    /// slopes that the heightmap interpolation can produce.
+    /// </summary>
+    public float StepUpHeight { get; set; } = 5.0f;
 
     public float Yaw { get; set; }
     public Vector3 Position { get; private set; }

src/AcDream.App/Rendering/GameWindow.cs

@@ -203,15 +203,14 @@ public sealed class GameWindow : IDisposable
                             float plocalX = playerEntity.Position.X - (plbX - _liveCenterX) * 192f;
                             float plocalY = playerEntity.Position.Y - (plbY - _liveCenterY) * 192f;
                             uint pinitCellId = ((uint)plbX << 24) | ((uint)plbY << 16) | 0x0001u;
-                            // Use the server-sent position directly — the server already
-                            // gave us a valid position at CreateObject time. Running it
-                            // through PhysicsEngine.Resolve was mapping the player into
-                            // an indoor cell (e.g., the foundry at Z=66) when they're
-                            // actually standing on outdoor terrain at Z=93+. The physics
-                            // engine's cell-containment check is too aggressive for the
-                            // initial placement. Once the player starts moving, Resolve
-                            // will keep them on the correct surface.
-                            _playerController.SetPosition(playerEntity.Position, pinitCellId & 0xFFFFu);
+                            // Resolve the initial position through the physics engine to
+                            // get the correct terrain Z. The server-sent Z may be stale
+                            // from a previous ACE relocation. With indoor transitions
+                            // disabled, Resolve will always snap to outdoor terrain Z.
+                            var initResult = _physicsEngine.Resolve(
+                                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
                             // rather than hardcoding. Extract yaw from the quaternion.
                             var q = playerEntity.Rotation;