feat: dedicated server security hardening

Comprehensive security system to protect against packet-sniffing attacks,
XUID harvesting, privilege escalation, bot flooding, and XUID impersonation.

- Stream cipher: per-session XOR cipher with 4-message handshake via
  CustomPayloadPacket (MC|CKey, MC|CAck, MC|COn). Negotiated per-connection,
  backwards compatible (old clients/servers fall back to plaintext).
- Security gate: buffers all game data until cipher handshake completes,
  preventing unsecured clients from receiving any XUIDs or game state.
- Cipher handshake enforcer: kicks clients that don't complete the handshake
  within 5 seconds (configurable via require-secure-client).
- Identity tokens: persistent per-XUID tokens in identity-tokens.json,
  issued over the encrypted channel, verified on reconnect. Prevents XUID
  replay attacks. Client stores server-specific tokens.
- PROXY protocol v1: parses real client IPs from playit.gg tunnel headers
  so rate limiting, IP bans, and XUID spoof detection work per-player.
- Rate limiting: per-IP sliding window (default 5 connections/30s) with
  pending connection cap (default 10).
- Privilege hardening: OP requires ops.json, live checks on every command
  and privilege packet. Host-only server settings changes.
- XUID stripping: PreLoginPacket response sends INVALID_XUID placeholders.
- Packet validation: readUtf global string cap, reduced max packet size,
  stream desync protection on oversized strings.
- OpManager: persistent ops.json with XUID-based OP list.
- Whitelist improvements: whitelist add accepts player names with ambiguity
  detection, XUID cache from login attempts.
- revoketoken command: revoke identity tokens for players who lost theirs.
- server.log: persistent log file written alongside console output with
  flush-per-write to survive crashes.
- CLI security logging: consolidated per-join security summary with cipher
  status, token status, XUID, and real IP. Security warnings for kicks,
  spoofing, and unauthorized commands.

Minecraft.Server/Security/CipherHandshakeEnforcer.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#ifdef _WINDOWS64
+#include <Windows.h>
+#endif
+
+#include <vector>
+
+namespace ServerRuntime
+{
+	namespace Security
+	{
+		/**
+		 * Tracks pending cipher handshakes and kicks clients that don't complete
+		 * within the grace period.
+		 *
+		 * When require-secure-client is enabled, old/unpatched clients that ignore
+		 * MC|CKey are disconnected before they receive any PlayerInfoPacket data
+		 * containing other players' XUIDs.
+		 *
+		 * Called from the main tick thread only (PlayerList::tick).
+		 */
+		class CipherHandshakeEnforcer
+		{
+		public:
+			// 5 seconds at 20 TPS. The security gate buffers all game data until
+			// cipher completes, so no data leaks regardless of grace period length.
+			// 5 seconds accommodates high-latency connections.
+			static const int kGraceTicks = 100;
+
+			CipherHandshakeEnforcer();
+			~CipherHandshakeEnforcer();
+
+			CipherHandshakeEnforcer(const CipherHandshakeEnforcer &) = delete;
+			CipherHandshakeEnforcer &operator=(const CipherHandshakeEnforcer &) = delete;
+
+			/**
+			 * Register that MC|CKey was sent to this smallId at the given tick.
+			 */
+			void OnCipherKeySent(unsigned char smallId, unsigned int currentTick);
+
+			/**
+			 * Check for timed-out handshakes. Returns smallIds that exceeded the
+			 * grace period without the cipher becoming active. Also returns
+			 * smallIds that just completed (cipher became active) in outCompleted.
+			 */
+			void CheckTimeouts(unsigned int currentTick,
+				std::vector<unsigned char> &outExpired,
+				std::vector<unsigned char> &outCompleted);
+
+			/**
+			 * Clean up tracking for a disconnected connection.
+			 */
+			void OnDisconnected(unsigned char smallId);
+
+		private:
+			static const int MAX_CONNECTIONS = 256;
+			unsigned int m_sentTick[MAX_CONNECTIONS]; // 0 = not tracked
+		bool m_tracked[MAX_CONNECTIONS];
+		};
+
+		CipherHandshakeEnforcer &GetHandshakeEnforcer();
+	}
+}