Add first-seen and last-seen DNS record tracking

[Ductrung03]

Fixes #1.
This adds DNS record tracking metadata to the JSON dataset:

• first-seen
• last-seen
• seen
  Behavior:
• New records initialize all tracking fields.
• Existing records preserve first-seen, update last-seen, and increment seen.
• Previous JSON output is merged into the new result so tracking persists across runs.
• Added --fs and --ls flags to filter first-seen and last-seen style messages.
  Validation:
• git diff --check
• C++17 syntax check passed for changed translation units.
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