[WIP] Review and document ICMP with ICMPv6 support

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>ICMP: review, ICMPv6, and documentation</issue_title>
<issue_description>## Problem

Magma.Internet.Icmp exists but is unvalidated and undocumented. ICMPv6 support status is unknown.

Sub-tasks

• Review existing ICMP implementation for correctness gaps
• Add ICMPv6 support if absent (Neighbour Discovery, Router Solicitation at minimum)
• Document ICMP usage patterns in docs/

Dependencies

Depends on: #109
Unlocks: #135 (ICMP ping sample)

Phase

Phase 1 (Core Protocol & Tooling)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ICMP: review, ICMPv6, and documentation #110

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