feat(net): Phase B.2 — MoveToState + AutonomousPosition message builders

Outbound GameAction message builders for player movement:
- MoveToState (0xF61C): sent on motion state changes (start/stop
  walking, turn, speed change). Carries RawMotionState (flag-driven
  variable fields) + WorldPosition + sequence numbers.
- AutonomousPosition (0xF753): periodic position heartbeat sent
  every ~200ms while moving. No RawMotionState — just WorldPosition
  + sequences + contact byte.

Both follow the GameAction envelope pattern (0xF7B1 + sequence +
action type) established by GameActionLoginComplete. Wire format
ported from references/holtburger movement protocol — field order
and alignment match exactly (contact byte + pad_to_4).

Also:
- Adds WriteFloat to PacketWriter (needed by both builders)
- Adds SendGameAction + NextGameActionSequence to WorldSession
  (public wrappers for PlayerMovementController in Task 2)

11 new tests, 265 total, all green.

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

src/AcDream.Core.Net/WorldSession.cs

@@ -126,6 +126,16 @@ public sealed class WorldSession : IDisposable
     /// </summary>
     private bool _loginCompleteSent;
 
+    /// <summary>
+    /// Phase B.2: per-session game-action sequence counter. Monotonically
+    /// incremented by <see cref="NextGameActionSequence"/> and embedded in
+    /// every outbound MoveToState / AutonomousPosition GameAction message.
+    /// ACE's GameActionPacket.HandleGameAction reads the sequence field but
+    /// currently only uses it for logging — however retail clients do
+    /// increment it, so we match that behaviour.
+    /// </summary>
+    private uint _gameActionSequence;
+
     public WorldSession(IPEndPoint serverLogin)
     {
         _loginEndpoint = serverLogin;
@@ -419,6 +429,24 @@ public sealed class WorldSession : IDisposable
         }
     }
 
+    /// <summary>
+    /// Phase B.2: send a pre-built GameAction body (which already contains
+    /// the 0xF7B1 envelope + sequence + action-type header). Used by the
+    /// PlayerMovementController for MoveToState and AutonomousPosition.
+    /// </summary>
+    public void SendGameAction(byte[] gameActionBody)
+    {
+        SendGameMessage(gameActionBody);
+    }
+
+    /// <summary>
+    /// Phase B.2: get and increment the game-action sequence counter.
+    /// Call once per outbound movement message; pass the returned value
+    /// to <see cref="Messages.MoveToState.Build"/> or
+    /// <see cref="Messages.AutonomousPosition.Build"/>.
+    /// </summary>
+    public uint NextGameActionSequence() => ++_gameActionSequence;
+
     private void SendGameMessage(byte[] gameMessageBody)
     {
         var fragment = GameMessageFragment.BuildSingleFragment(