Add DNS first-seen and last-seen history

[clynbmilio]

Closes #1.

Summary:

• add DNS record history fields to JSON output: first-seen, last-seen, seen, and currently_seen
• carry history forward when a previous dooked JSON scan is used as input
• preserve DNS records that disappeared from the current scan so rotating/load-balanced records are not lost immediately
• add --fs, --ls, and --lsd comparison flags for first-seen and last-seen reporting
• document the new history output and flags in README.md
• fix two small build compatibility issues found while validating on current Ubuntu/Boost: Boost Beast header value conversion and a missing include

Validation:

• git diff --check
• submodules initialized successfully
• fork GitHub Actions build check passed: https://github.com/clynbmilio/dooked/actions/runs/27055020294

Bounty / payment:

• If this PR is selected and merged for the $100 bounty, PayPal payout can go to versytwo@icloud.com