fix(camera): smooth chase-camera Z follow so the jump arc is visible on screen

Diagnostic from K-fix10 confirmed our local jump physics is
mathematically perfect — every full-charge jump produces
formulaPeak = actualPeakDz = vz²/19.6 to four-digit precision
(3.11 m for Jump skill 208). Yet the user observed retail
clients seeing the SAME character jump much higher than ACdream
sees of itself.

Root cause: ChaseCamera tracked player.Z 1:1. When the player
rises 3 m the camera rises 3 m too — the player's screen
position never changes during the arc, so the jump is visually
invisible. Retail's chase camera lags the Z follow, so an
observer sees the player visibly rise on screen.

Fix: low-pass filter the camera's Z target.
ChaseCamera.Update gains a dt parameter and an exponential
smoother:
    alpha = 1 - exp(-dt / ZFollowTimeConstant)
    smoothedZ += (player.Z - smoothedZ) * alpha
ZFollowTimeConstant defaults to 0.15 s — slow enough that a
~1 s jump arc shows up clearly on screen, fast enough that
slope walking still feels glued. The look-at point still uses
the raw player Z so the camera tilts up to keep the airborne
character in frame.

Drive-by: stripped K-fix10 jump diagnostic logging now that the
math has been confirmed correct.

Tests stay 1222 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -3987,8 +3987,11 @@ public sealed class GameWindow : IDisposable
                 _worldState.RelocateEntity(pe, currentLb);
             }
 
-            // Update chase camera.
-            _chaseCamera.Update(result.Position, _playerController.Yaw);
+            // Update chase camera. K-fix11 (2026-04-26): pass dt so the
+            // Z-smoothing low-pass filter tracks correctly on variable
+            // frame rates — without this the camera tracks player.Z 1:1
+            // and jumps look invisible (camera rises with player).
+            _chaseCamera.Update(result.Position, _playerController.Yaw, (float)dt);
 
             // Send outbound movement messages to the live server.
             if (_liveSession is not null)