docs(lab4): submit Lab 4. add network trace and TLS decode

[linxel]

Goal

Complete Lab 4 — OS & Networking: trace TCP packets, debug a broken deploy, and decode TLS handshake.

Changes

• Add submissions/lab4.md with packet capture analysis
• Add five debugging commands output (ss, ip route, mtr, dig, journalctl)
• Add 502 error debugging reflection
• Add outside-in debugging chain for broken deploy
• Add mini-postmortem for "bind: address already in use"
• Add Bonus TLS handshake decode with screenshot

Testing

• Verified with local QuickNotes running on :8080
• Captured traffic with tcpdump on lo interface
• Tested broken deploy with two instances on same port
• Verified TLS handshake with Caddy reverse proxy on port 8443

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed
• submissions/lab4.md updated