Implement IPv6 Neighbor Discovery basics #190

Summary

Add the IPv6 Neighbor Discovery mechanisms required to resolve link-layer addresses and maintain basic on-link neighbor state.

Scope

• Parse and validate ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages.
• Implement neighbor-cache states, expiration, and bounded capacity.
• Validate hop limit, message lengths, and relevant options before updating cache entries.
• Define behavior for duplicate, stale, and unreachable neighbor entries.
• Integrate with the existing network packet and interface abstractions.

Acceptance criteria

• Unit tests cover valid messages, truncated packets, malformed options, and cache transitions.
• Invalid packets never mutate neighbor-cache state.
• Cache growth and retry behavior are bounded.
• Document the initial IPv6 features supported and those still out of scope.

Engineering constraints

• Keep the kernel Rust-first, no_std, and x86_64-compatible.
• Integrate with existing Rustalis subsystem interfaces; do not create a parallel implementation.
• Document ownership, initialization order, failure behavior, and any architecture-specific unsafe invariants.
• Add host-side tests for hardware-independent logic and QEMU coverage for hardware paths where practical.
• Do not claim verification that has not actually been run.