feat(net): track + echo movement sequence counters

Sprint 1a of the audit remediation plan.

Extracts the 4 movement sequence counters from inbound server messages
and echoes them in outbound MoveToState + AutonomousPosition instead
of hardcoded zeros:

- instanceSequence (slot 8 in CreateObject PhysicsData timestamps)
- teleportSequence (slot 4, also from PlayerTeleport 0xF751)
- serverControlSequence (slot 5)
- forcePositionSequence (slot 6, also from UpdatePosition 0xF748)

Source: holtburger player/types.rs:237-245, mutations.rs:182-706.
The server uses these to detect stale/reordered movement packets.
Previously all zeros → server couldn't distinguish epoch boundaries.

Changes:
- CreateObject.Parsed: +4 sequence fields extracted from timestamps
- UpdatePosition.Parsed: +3 sequence fields from trailing u16s
- WorldSession: tracks 4 counters, updates from CreateObject/
  UpdatePosition/PlayerTeleport for the player's own GUID
- GameWindow: passes tracked values to MoveToState.Build and
  AutonomousPosition.Build

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

src/AcDream.Core.Net/WorldSession.cs

@@ -116,6 +116,12 @@ public sealed class WorldSession : IDisposable
     public event Action<State>? StateChanged;
 
     public State CurrentState { get; private set; } = State.Disconnected;
+
+    /// <summary>Movement sequence counters for outbound MoveToState/AutonomousPosition.</summary>
+    public ushort InstanceSequence => _instanceSequence;
+    public ushort ServerControlSequence => _serverControlSequence;
+    public ushort TeleportSequence => _teleportSequence;
+    public ushort ForcePositionSequence => _forcePositionSequence;
     public CharacterList.Parsed? Characters { get; private set; }
 
     private readonly NetClient _net;
@@ -129,6 +135,16 @@ public sealed class WorldSession : IDisposable
     private uint _clientPacketSequence;
     private uint _fragmentSequence = 1;
 
+    // Movement sequence counters — echoed back in every MoveToState and
+    // AutonomousPosition so the server can detect stale/reordered packets.
+    // Initialized from CreateObject PhysicsData timestamps, updated by
+    // UpdatePosition/UpdateMotion/PlayerTeleport. Per holtburger:
+    // instance=slot 8, teleport=slot 4, serverControl=slot 5, forcePosition=slot 6.
+    private ushort _instanceSequence;
+    private ushort _serverControlSequence;
+    private ushort _teleportSequence;
+    private ushort _forcePositionSequence;
+
     // Phase A.3: background receive thread buffers raw UDP datagrams into
     // a channel so the render thread never blocks on socket I/O.
     private readonly Channel<byte[]> _inboundQueue =
@@ -400,6 +416,15 @@ public sealed class WorldSession : IDisposable
                 var parsed = CreateObject.TryParse(body);
                 if (parsed is not null)
                 {
+                    // Initialize sequence counters from the player's own CreateObject.
+                    if (parsed.Value.Guid == Characters?.Characters.FirstOrDefault().Id)
+                    {
+                        _instanceSequence      = parsed.Value.InstanceSequence;
+                        _teleportSequence      = parsed.Value.TeleportSequence;
+                        _serverControlSequence = parsed.Value.ServerControlSequence;
+                        _forcePositionSequence = parsed.Value.ForcePositionSequence;
+                    }
+
                     EntitySpawned?.Invoke(new EntitySpawn(
                         parsed.Value.Guid,
                         parsed.Value.Position,
@@ -440,6 +465,14 @@ public sealed class WorldSession : IDisposable
                 var posUpdate = UpdatePosition.TryParse(body);
                 if (posUpdate is not null)
                 {
+                    // Update sequence counters from the player's own position updates.
+                    if (posUpdate.Value.Guid == Characters?.Characters.FirstOrDefault().Id)
+                    {
+                        _instanceSequence      = posUpdate.Value.InstanceSequence;
+                        _teleportSequence      = posUpdate.Value.TeleportSequence;
+                        _forcePositionSequence = posUpdate.Value.ForcePositionSequence;
+                    }
+
                     PositionUpdated?.Invoke(new EntityPositionUpdate(
                         posUpdate.Value.Guid,
                         posUpdate.Value.Position,
@@ -457,10 +490,11 @@ public sealed class WorldSession : IDisposable
                 // movement; the LoginComplete is sent from GameWindow once
                 // the destination UpdatePosition is received and the player
                 // has been snapped to the new cell.
-                uint sequence = 0;
+                ushort sequence = 0;
                 if (body.Length >= 6)
                     sequence = System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(
                         body.AsSpan(4, 2));
+                _teleportSequence = sequence;  // track for outbound movement messages
                 TeleportStarted?.Invoke(sequence);
             }
         }