Add DHCPv6 listener and address method inference

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Consolidated from NEXT.md Phase 2.

Summary

Add DHCPv6 capture and use it to improve address-method inference.

Priority / Effort

• Priority: Medium
• Effort: Medium

Files

• lib/dhcpv6_listener.go (new)
• lib/ndp_stats.go
• main.go
• lib/display.go

Implementation

• Capture DHCPv6 traffic on ports 546/547 and multicast group ff02::1:2
• Parse core DHCPv6 message types
• Correlate leases/DUID with peers and router flags
• Add config-method field: SLAAC, SLAAC-Privacy, DHCPv6, Unknown

Acceptance Criteria

• DHCPv6 server/client activity is visible in stats
• Peer config method reflects correlation logic with a clear fallback behavior

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Change the peers table to be NDP Peers and add a new one for DHCP Peers