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; }