fix(app+core): Phase B.3 — streaming follows player, AC jump physics

Three fixes for user-reported movement bugs:

1. Character disappears far from spawn: streaming observer now computed
   from _playerController.Position when player mode is active, instead
   of _lastLivePlayerLandblockId which only updates from server echoes
   (never for autonomous moves). The 5x5 streaming window now follows
   the player as they walk.

2. Jump physics from ACE: JumpImpulse=5.0 and GravityAccel=9.8
   matching AC's formula: velocity_z = sqrt(height * 19.6) where
   height = BurdenMod * (JumpSkill / (JumpSkill + 1300) * 22.2 + 0.05)
   For a new char (skill=100, burden=50%): height≈1.31, vz≈5.07.

3. Gravity reduced from 20 to 9.8 (AC's F_GRAVITY constant).

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -64,8 +64,14 @@ public sealed class PlayerMovementController
 
     public float VerticalVelocity { get; private set; }
     public bool IsAirborne { get; private set; }
-    public float JumpImpulse { get; set; } = 3.5f;  // placeholder; retail scales by Jump skill
+    /// <summary>
-    public float GravityAccel { get; set; } = 20f;
+    /// Jump velocity Z. AC formula: sqrt(height * 19.6) where
+    /// height = BurdenMod * (JumpSkill / (JumpSkill + 1300) * 22.2 + 0.05) * power.
+    /// For a typical new char (skill=100, burden=50%, power=1.0) ≈ 5.07.
+    /// </summary>
+    public float JumpImpulse { get; set; } = 5.0f;
+    /// <summary>AC's gravity constant (F_GRAVITY = 9.8 m/s²).</summary>
+    public float GravityAccel { get; set; } = 9.8f;
     public float AirControlFactor { get; set; } = 0.2f;
 
     /// <summary>

src/AcDream.App/Rendering/GameWindow.cs

@@ -1562,11 +1562,22 @@ public sealed class GameWindow : IDisposable
             int observerCx = _liveCenterX;
             int observerCy = _liveCenterY;
 
-            if (_liveSession is not null
+            if (_playerMode && _playerController is not null)
+            {
+                // Player mode: follow the physics-resolved player position.
+                // The player walks via the local physics engine; the server
+                // doesn't echo back our autonomous moves, so _lastLivePlayer*
+                // stays at the login position. Compute the landblock from the
+                // controller's current world-space position instead.
+                var pp = _playerController.Position;
+                observerCx = _liveCenterX + (int)System.Math.Floor(pp.X / 192f);
+                observerCy = _liveCenterY + (int)System.Math.Floor(pp.Y / 192f);
+            }
+            else if (_liveSession is not null
                 && _liveSession.CurrentState == AcDream.Core.Net.WorldSession.State.InWorld
                 && _lastLivePlayerLandblockId is { } lid)
             {
-                // Live mode: follow the server's last-known player position.
+                // Live mode (fly camera): follow the server's last-known player position.
                 observerCx = (int)((lid >> 24) & 0xFFu);
                 observerCy = (int)((lid >> 16) & 0xFFu);
             }