feat(net): plumb IsGrounded through EntityPositionUpdate (L.3.2 Task 2)

PositionFlags.IsGrounded (0x04) was already parsed by UpdatePosition
but not exposed through the Parsed record or EntityPositionUpdate.
Adds the bool field to both records so OnLivePositionUpdated can
consume it for retail-faithful MoveOrTeleport routing
(acclient @ 0x00516330: has_contact=false → no-op during airborne arc).

Consumed in subsequent task (L.3.1+L.3.2 Task 3).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.Core.Net/Messages/UpdatePosition.cs

@@ -64,6 +64,7 @@ public static class UpdatePosition
         CreateObject.ServerPosition Position,
         System.Numerics.Vector3? Velocity,
         uint? PlacementId,
+        bool IsGrounded,
         ushort InstanceSequence = 0,
         ushort TeleportSequence = 0,
         ushort ForcePositionSequence = 0);
@@ -164,6 +165,7 @@ public static class UpdatePosition
                 RotationW: rw, RotationX: rx, RotationY: ry, RotationZ: rz);
 
             return new Parsed(guid, serverPos, velocity, placementId,
+                IsGrounded: (flags & PositionFlags.IsGrounded) != 0,
                 instSeq, teleSeq, forceSeq);
         }
         catch