Add Peer to Peer communication

New Features:

• Peer-to-peer messaging API: New sendPeerMessage(targetClientId, message) method for direct client-to-client communication
• Security restriction: Only clients with permanent IDs (1-100, registered with serial numbers) can send/receive peer messages
• ID range separation:
  • IDs 1-100: Permanent IDs for registered clients (supports P2P)
  • IDs 101-254: Temporary IDs for unregistered clients (pub/sub only)
• New message type: CAN_PS_PEER_MSG (0x09) protocol command

Implementation Changes:

• Added handlePeerMessage() in broker to validate and forward peer messages
• Added _nextTempID field to track temporary ID assignments (starts at 101)
• Enhanced onDirectMessage() callback to handle both broker and peer messages
• Extended frame support for long peer messages

Documentation:

• New PEER_TO_PEER.md guide explaining P2P architecture and usage
• Updated PUBSUB_API.md with sendPeerMessage() documentation
• Updated SERIAL_NUMBER_MANAGEMENT.md with ID range table and P2P capabilities
• Updated QUICK_REFERENCE.md with unified messaging commands

Examples:

• New PeerToPeer.ino example demonstrating direct client-to-client messaging
• Updated ClientWithSerial.ino with unified msg:id:message command supporting both broker and peer messages

Protocol Enhancements:

• Broker validates sender and receiver have permanent IDs before forwarding
• Serial number verification in ID responses to prevent ID conflicts
• Improved subscription cleanup when clients disconnect