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/Packets/PacketWriter.cs

@@ -88,6 +88,13 @@ public sealed class PacketWriter
         _position += bytes.Length;
     }
 
+    public void WriteFloat(float value)
+    {
+        EnsureCapacity(4);
+        BinaryPrimitives.WriteSingleLittleEndian(_buffer.AsSpan(_position), value);
+        _position += 4;
+    }
+
     /// <summary>Pad with zeros so the buffer length is a multiple of 4.</summary>
     public void AlignTo4()
     {