Add OUI/MAC vendor lookup

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Consolidated from NEXT.md Phase 1.

Summary

Add OUI/MAC vendor lookup so peers can be enriched with manufacturer data.

Priority / Effort

• Priority: Medium
• Effort: Low

Files

• lib/oui.go (new)
• lib/ndp_stats.go
• lib/display.go

Implementation

• Add a local OUI prefix map and lookup helper
• Add vendor to peer summary and detail display

Acceptance Criteria

• Known OUIs resolve to vendor names
• Unknown OUIs remain blank or Unknown without errors