fix(movement): jump works locally (airborne velocity preserved)

Two fixes for jump physics:
- Skip ground-snap when velocity Z > 0 (prevents immediate re-landing
  at high framerates where per-frame Z delta < 0.05 snap threshold)
- Guard apply_current_movement velocity write behind OnWalkable check
  (prevents MotionInterpreter.DoMotion from zeroing jump velocity on
  every frame while airborne)
- Guard PlayerMovementController velocity replacement behind OnWalkable
  (preserves momentum during airborne flight)

Jump works locally but server packet not yet sent (BUG-002).
Facing direction mismatch logged as BUG-003.
RunRate not verified as BUG-004.

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

src/AcDream.Core/Physics/MotionInterpreter.cs

@@ -539,8 +539,17 @@ public sealed class MotionInterpreter
         if (InterpretedState.ForwardCommand == MotionCommand.RunForward)
             MyRunRate = InterpretedState.ForwardSpeed;
 
-        var localVelocity = get_state_velocity();
-        PhysicsObj.set_local_velocity(localVelocity);
+        // Only replace velocity when grounded. While airborne, the physics
+        // body's integrated velocity (from LeaveGround) should persist —
+        // gravity pulls Z down, horizontal momentum is preserved.
+        // The retail client's apply_current_movement also gates on Contact+OnWalkable
+        // before writing velocity (the full decompiled flow routes through
+        // update_object which checks transient state).
+        if (PhysicsObj.OnWalkable)
+        {
+            var localVelocity = get_state_velocity();
+            PhysicsObj.set_local_velocity(localVelocity);
+        }
     }
 
     // ── FUN_00529390 — jump ───────────────────────────────────────────────────