Add ISOBUS (ISO 11783 / SAE J1939) protocol support

[Copilot]

Adds a new contrib module implementing the ISOBUS protocol (ISO 11783), the CAN-based communication standard for agricultural and forestry machinery built on SAE J1939.

New module: scapy/contrib/automotive/isobus.py

Core layer:

• ISOBUS — Extends CAN, replacing the 29-bit identifier with individual J1939 fields (priority, reserved, data_page, pdu_format, pdu_specific, source_address). Computed pgn property handles both PDU1 (peer-to-peer, pdu_format < 0xF0) and PDU2 (broadcast, pdu_format >= 0xF0) addressing correctly.

PGN packet classes with bind_layers dispatch:

Class	PGN
ISOBUSAddressClaimed	0x00EE00
ISOBUSRequestForPGN	0x00EA00
ISOBUSAcknowledgment	0x00E800
ISOBUSTransportProtocolCM	0x00EC00
ISOBUSTransportProtocolDT	0x00EB00
ISOBUSWorkingSetMaster	0x00E600
ISOBUSWorkingSetMember	0x00E700
ISOBUSCommandedAddress	0x00FED8

ISOBUSTransportProtocolCM uses ConditionalField to handle all five TP.CM variants (BAM, RTS, CTS, EOM ACK, Abort) in a single class.

ISOBUSAddressClaimed stores the 8-byte little-endian J1939 NAME as a LELongField with read properties for all sub-fields (identity_number, manufacturer_code, industry_group, device_class, etc.).

Helper: build_isobus_name() constructs the 64-bit NAME integer from named component fields.

from scapy.contrib.automotive.isobus import *

# Address Claiming
nn = build_isobus_name(identity_number=0x001, manufacturer_code=0x123,
                       industry_group=2, device_class=4)
pkt = ISOBUS(priority=6, pdu_format=0xEE, pdu_specific=0xFF,
             source_address=0x80, length=8) / ISOBUSAddressClaimed(node_name=nn)

# Auto-dissection by pdu_format
parsed = ISOBUS(bytes(pkt))
assert isinstance(parsed.payload, ISOBUSAddressClaimed)
assert parsed.payload.industry_group == 2
assert parsed.pgn == 0x00EE00

# Multi-packet BAM
pkt = ISOBUS(priority=7, pdu_format=0xEC, pdu_specific=0xFF,
             source_address=0x80, length=8) / \
      ISOBUSTransportProtocolCM(control_byte=32, total_message_size=20,
                                total_number_of_packets=3, pgn=0x00FED8)

Tests: test/contrib/automotive/isobus.uts

33 regression tests covering build/dissect round-trips, PGN computation, NAME subfield access, all TP.CM variants, and automatic layer dispatch.

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