fix(physics): jump apex velocity zeroing bug

SmallVelocity threshold (0.25 m/s) in UpdatePhysicsInternal was zeroing
velocity every frame while airborne at the jump apex. With vel~0.01 m/s
and gravity adding only 0.012/frame, the zeroing won every frame and
the character got stuck at peak height forever.

Fix: only apply small-velocity zeroing when OnWalkable (grounded).
While airborne, gravity must accumulate freely through the zero-crossing.

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

src/AcDream.Core/Physics/PhysicsBody.cs

@@ -298,7 +298,10 @@ public sealed class PhysicsBody
             calc_friction(dt, velocityMag2);
 
             // If velocity fell below the "small" threshold after friction, stop.
-            if (velocityMag2 - SmallVelocitySquared < 0.0002f)
+            // Only apply when grounded — while airborne, gravity must accumulate
+            // even when velocity is near zero (e.g., at jump apex).
+            if (velocityMag2 - SmallVelocitySquared < 0.0002f
+                && TransientState.HasFlag(TransientStateFlags.OnWalkable))
                 Velocity = Vector3.Zero;
 
             // Euler integration: position += v*dt + 0.5*a*dt²