docs(lab4): packet capture trace, debugging, outside-in debug chain, TLS 1.3 handshake via Caddy

[blacktree-lab]

Goal

Trace a live HTTP request end-to-end with tcpdump, walk an outside-in debug chain on a broken deploy, and decode a TLS 1.3 handshake via a Caddy reverse proxy.

Changes:

• Added submissions/lab4.md with annotated packet capture, five debugging command outputs, 502 reflection, outside-in debug chain, blameless postmortem, and TLS handshake decode
• Added lab4-trace.txt with full tcpdump output of the POST /notes request captured on loopback

Testing:

• Captured TCP three-way handshake, HTTP POST /notes request, 201 Created response, and graceful FIN teardown in lab4-trace.pcap
• Ran all five debugging commands (ss, ip route, mtr, dig, journalctl) and documented output + reasoning
• Reproduced port conflict (bind: address already in use) by launching two QuickNotes instances on :8080
• Walked outside-in chain (ps -> ss -> curl -> iptables -> dig), identified root cause, killed conflicting process, and re-verified health
• Captured TLS 1.3 handshake via Caddy reverse proxy on :8443; confirmed TLS_AES_128_GCM_SHA256 / X25519 negotiation
• Extracted full three-cert chain (leaf + ECC intermediate + ECC root) with openssl s_client -servername localhost

Checklist:

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/lab4.md updated
• lab4-trace.txt included alongside submission