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.Client/PlayerConnection.h

@@ -137,6 +137,21 @@ public:
 	// 4J Added
 	bool hasClientTickedOnce() { return m_bHasClientTickedOnce; }
 
+	// Identity token verification state (accessed from both recv and main threads)
+	std::atomic<bool> m_identityVerified;
+	std::atomic<int> m_identityChallengeTick;
+
+	// Security gate: buffer packets until cipher handshake completes
+	bool m_securityGateOpen;
+	vector<shared_ptr<Packet>> m_securityBuffer;
+
+	bool isIdentityVerified() const { return m_identityVerified; }
+	int getIdentityChallengeTick() const { return m_identityChallengeTick; }
+	void setIdentityChallengeTick(int tick) { m_identityChallengeTick = tick; }
+	void setIdentityVerified(bool v) { m_identityVerified = v; }
+	bool isSecurityGateOpen() const { return m_securityGateOpen; }
+	void openSecurityGate();
+
 private:
 	bool m_bCloseOnTick;
 	vector<wstring> m_texturesRequested;