Feature/lab4 – Complete lab4 OS and networking trace

[kujifined]

Goal

Complete Lab 4 by tracing a QuickNotes request end-to-end and documenting outside-in debugging of a broken local deployment.

Changes

• Added submissions/lab4.md with the full Lab 4 report.
• Captured and annotated a POST /notes request, including TCP handshake, HTTP request, HTTP response, and graceful connection close.
• Documented debugging command outputs, using macOS equivalents for Linux-only tools where needed.
• Reproduced a broken deployment caused by bind: address already in use.
• Added an outside-in debugging chain with command outputs, decisions, root cause, and a blameless mini-postmortem.

Testing

Verified locally on macOS Darwin by:

• Running QuickNotes on localhost:8080.
• Sending a POST /notes request with curl -v.
• Capturing loopback traffic with tcpdump on lo0.
• Decoding the capture with tcpdump -r.
• Checking that QuickNotes listens on port 8080 with lsof.
• Verifying /health returns 200.
• Reproducing the port conflict failure by starting a second QuickNotes instance on :8080.
• Repairing the issue by killing the conflicting process and restarting QuickNotes.

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/labN.md updated