feat(net): Implement Display for ICMP Error messages inner packet

[qmonnet]

While working on ICMP Error tests, I noticed I couldn't easily observe the headers for the inner packets embedded within the error messages. Implement trait Display for the types involved in inner packets embedded within ICMP Error messages, so that we can observe the inner headers as well with the packet dumper.

[Copilot]

Pull request overview

This PR extends packet dumping so ICMP error messages can display embedded inner packet headers, including newly exposed accessors needed by the display code.

Changes:

• Adds Display implementations for embedded/truncated ICMP, TCP, UDP, and embedded header containers.
• Implements ICMPv6 display formatting.
• Widens selected constants/accessors to pub(crate) for display support.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
net/src/packet/display.rs	Adds display formatting for ICMPv6 and embedded/truncated inner headers.
net/src/headers/embedded_view.rs	Makes embedded header accessors crate-private for reuse.
net/src/icmp4/truncated.rs	Exposes truncated ICMPv4 type for display formatting.
net/src/icmp6/truncated.rs	Exposes truncated ICMPv6 type for display formatting.
net/src/tcp/truncated.rs	Exposes TCP truncated minimum header length for display formatting.
net/src/udp/truncated.rs	Exposes UDP truncated minimum header length for display formatting.