Add historic connections toggle

[domcyrus]

Summary

• Press t to toggle display of recently closed connections alongside active ones
• Closed connections are archived to a separate store (capped at 5,000, oldest evicted first)
• Historic connections rendered in dimmed DarkGray styling in both flat and grouped views
• Stats panel counts only active connections; details tab shows closed-at timing
• Integrates with r (reset), x (clear), and filter/sort

Test plan

• Run rustnet, wait for connections to close, press t — closed connections reappear dimmed
• Press t again — historic connections hidden
• Verify scrolling works correctly in both flat and grouped views
• Verify x clears historic connections, r resets the toggle
• Verify stats panel shows only active connection counts
• Verify details tab shows "Historic Connection" title and "Closed (Xs ago)" for historic entries