Function decodeJ1939

Decode a CAN frame as a J1939 message

Extracts:

• Priority, PGN, source address
• PGN name from database (if available)
• Raw data payload

Example: decodeJ1939({ id: 0x18F00401, data: [0x10, 0x20, ...] }) → { priority: 3, pgn: 0xF004, sa: 0x01, name: "Engine Speed", ... }