Add first-seen and last-seen DNS history

[nexiumbiz-debug]

Summary

• add first-seen, last-seen, and seen counters to each DNS record in JSON output
• preserve previously seen DNS records in later JSON outputs when they disappear from the current scan, so rotating/load-balanced responses can be tracked over time
• add comparison flags:
  • --fs / --first-seen for first-seen records
  • --ls / --last-seen-days for records missing for at least N days
  • --lsd / --last-seen-date for records missing since a US-formatted cutoff date
• document the new history-tracking flags in the README

Notes

This keeps older JSON input compatible by reading legacy first_seen / last_seen keys too, while writing the requested first-seen / last-seen keys going forward.

For the bounty payout: I am not posting payment details publicly. If this PR is selected, payout details can be provided privately through the maintainer's preferred trusted channel.

Validation

• git diff --check

Not run locally: build/tests. This Windows environment does not currently have cmake, cl, g++, or clang++ available on PATH.

Closes #1

[nexiumbiz-debug]

Just checking whether this is still useful for the first-seen/last-seen bounty. The PR is mergeable from GitHub's side, and I can adjust the JSON shape or flag behavior if you'd prefer it closer to the wording in #1. I haven't put payout details in the public thread, but can provide them privately if this is selected.

[nexiumbiz-debug]

I pushed a small follow-up at 4b6fb4c while rechecking the history comparison path. It replaces the two invalid binary_search calls with direct DNS record matching, so the first-seen/last-seen comparison does not depend on a strict ordering comparator that was not actually strict. The branch is still clean and mergeable from GitHub's side.