Add DNS history fields and prompts

[yy1142274323]

Summary

Adds DNS history metadata and prompt flags for issue #1.

Changes

• Writes first-seen, last-seen, and seen metadata for DNS records in JSON output.
• Reads existing history metadata from previous JSON results.
• Preserves older DNS records in the output dataset even when they are not visible in the current scan, which helps with load-balanced records that rotate between captures.
• Adds --fs to report records first seen in the current run.
• Adds --ls <days> to report records that are missing from the current run and have not been seen within the requested number of days.
• Adds --lsd <MM/DD/YYYY> to report records that are missing from the current run and have not been seen since the requested US-format date.
• Documents the history fields and flags in the README.

Testing

• git diff --check
• Manual static review of JSON read/write and CLI flag paths.
• Not run locally: CMake/build execution. This Windows host does not have cmake, g++, clang++, or MSVC cl available.

Payment

Payment details can be provided privately after review/merge.