Add script to export NebulaPacket topics from ROS2 bags to PCAP

[manato]

Motivation

• Provide a standalone tool to share Nebula LiDAR packet captures with non-ROS consumers by converting nebula_msgs/msg/NebulaPacket and nebula_msgs/msg/NebulaPackets topics from ROS 2 bags into classic .pcap files.
• Allow downstream tools to receive synthesized Ethernet/IPv4/UDP frames with configurable addresses, ports, MACs, and optional UDP checksum computation because packet payloads lack original transport metadata.

Description

• Add scripts/nebula_rosbag_to_pcap.py, a CLI script that opens a ROS 2 bag with rosbag2_py, deserializes target topic messages, and exports each NebulaPacket.stamp as the PCAP packet timestamp.
• Reconstruct synthetic network layers by building UDP segments, IPv4 headers (with computed IPv4 checksum), and Ethernet frames, then write a classic PCAP file with little-endian microsecond resolution and DLT_EN10MB link type via the PcapWriter class.
• Provide CLI options including --list-topics, --src-ip, --dst-ip, --src-port, --dst-port, --src-mac, --dst-mac, and --udp-checksum (choices zero or compute), with validation for ports and MAC parsing.
• Track conversion statistics (messages read, packets exported, empty/dropped packets, non-monotonic timestamps) and refuse to finish with zero exported packets.

Example usage

# Find candidate Nebula packet topics in a bag.
python3 scripts/nebula_rosbag_to_pcap.py \
  --bag /path/to/rosbag \
  --list-topics

# Export a selected topic to PCAP with the default LiDAR-oriented network
# profile.
python3 scripts/nebula_rosbag_to_pcap.py \
  --bag /path/to/rosbag \
  --topic /sensing/lidar/front/nebula_packets \
  --output /tmp/front_lidar.pcap

Testing

• No automated tests were added or executed for this change.

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Codex Task

[Copilot]

Pull request overview

Adds a standalone CLI tool to export Nebula LiDAR packet topics from a ROS 2 bag into a classic Ethernet (DLT_EN10MB) .pcap, synthesizing Ethernet/IPv4/UDP headers around each NebulaPacket.data payload and timestamping packets with NebulaPacket.stamp.

Changes:

• Introduces scripts/nebula_rosbag_to_pcap.py to read a ROS 2 bag via rosbag2_py, deserialize nebula_msgs/msg/NebulaPacket(s), and emit a classic PCAP file.
• Implements packet/frame construction (UDP, IPv4 with header checksum, Ethernet) plus optional UDP checksum computation and basic conversion statistics reporting.

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