Add dedicated prefix/subnet view

[Splat]

Consolidated from NEXT.md Phase 2.

Summary

Add a dedicated prefix/subnet view built from RA prefixes and observed addresses.

Priority / Effort

• Priority: Medium
• Effort: Low-Medium

Files

• lib/ndp_stats.go
• lib/display.go

Implementation

• Build a global prefix index from RA prefixes and observed addresses
• Add a prefix table view with host count and advertising routers

Acceptance Criteria

• Prefix view shows unique prefixes with counts and timestamps
• Prefix data remains pruned with window rules