Harden the mod against hostile servers and players

- PacketReader for all network data: lengths are checked against the real data
  (Mono allocated the claimed size first - a fake 2 GB length crashed the game)
- FsmSync: remote players can no longer switch arbitrary FSMs - never PLAYER /
  Systems / MSCO objects, CLICK only on FSMs that wait for a click, bolt/assembly
  events only along a transition the FSM really has for that event
- Time/weather (msco.world) and '* ' system-style chat only from the host in P2P
  (the dedicated server filters them itself); other msco.* ids are reserved

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

PacketReader.cs

@@ -0,0 +1,37 @@
+using System.IO;
+
+namespace MSCO
+{
+    /// <summary>
+    /// BinaryReader für Daten aus dem Netz (Server, Mitspieler). Mono legt bei ReadBytes(n) und ReadString() vorab ein
+    /// Array bzw. einen StringBuilder in der angegebenen Länge an - eine gefälschte Länge (z.B. 2 Milliarden) ließ das
+    /// Spiel mit "Out of Memory" abstürzen. Hier wird jede Länge gegen die tatsächlich vorhandenen Bytes geprüft.
+    /// </summary>
+    public sealed class PacketReader : BinaryReader
+    {
+        public PacketReader(byte[] data) : base(new MemoryStream(data ?? new byte[0], false)) { }
+
+        long Remaining { get { return BaseStream.Length - BaseStream.Position; } }
+
+        public override byte[] ReadBytes(int count)
+        {
+            if (count < 0 || count > Remaining) throw new EndOfStreamException("Invalid length " + count);
+            return base.ReadBytes(count);
+        }
+
+        public override char[] ReadChars(int count)
+        {
+            if (count < 0 || count > Remaining) throw new EndOfStreamException("Invalid length " + count);
+            return base.ReadChars(count);
+        }
+
+        public override string ReadString()
+        {
+            long start = BaseStream.Position;
+            int length = Read7BitEncodedInt();
+            if (length < 0 || length > Remaining) throw new EndOfStreamException("Invalid string length " + length);
+            BaseStream.Position = start;
+            return base.ReadString();
+        }
+    }
+}