Improved registration and corruption detection

Improvements Made:

1. CRC8 Checksum Validation

• Added calculateCRC8() function using CRC8-CCITT polynomial
• Clients append checksum to serial number when requesting registration
• Broker validates checksum before processing - silently rejects corrupted messages
• Prevents corrupted data from being stored

2. Message Length Validation

• Added isValidMessageLength() helper to validate message size
• Rejects messages that are too short or too long (outside expected range)
• Prevents buffer overflow and malformed data processing

3. Enhanced Registration Flow

• Broker side: Reads message into buffer, validates checksum, then validates serial format via callback
• Client side: Calculates checksum before sending registration request
• Messages that fail checksum validation are silently discarded (no invalid ID assigned)

4. Backward Compatibility

• Checksum feature controlled by CAN_PS_USE_CHECKSUM flag
• Can be disabled if needed for compatibility with older clients
• When disabled, falls back to serial validation only

Key Benefits:

• Detects corruption immediately - No invalid data reaches storage
• Silent rejection - Corrupted messages don't trigger responses, reducing bus congestion
• Low overhead - CRC8 is fast and only adds 1 byte per message
• Combined with existing protections: Validation callback + startup cleanup + deterministic delays + exponential backoff

The library now has comprehensive collision prevention:

✓ Deterministic delays (spacing between clients)
✓ Checksum validation (detect corruption)
✓ Serial format validation (application-specific via callback)
✓ Length validation (reject malformed messages)
✓ Smallest ID assignment (optimize ID usage)