Add first-seen and last-seen DNS history metadata

[LAieh12]

Fixes #1.

Implements first-seen / last-seen tracking for DNS records in JSON history output, including preservation of historical records that disappear from later scans.

Changes:

• Adds per-record first-seen, last-seen, seen, and currently-seen metadata to dns_probe entries.
• Preserves historical DNS records that are missing from the current run so old rotating/load-balanced answers are not lost.
• Marks preserved historical records as currently-seen: false while current records are written as currently-seen: true.
• Keeps default compare noise low by comparing against only records that were active in the previous run, while still allowing historical missing-record reporting through the new last-seen flags.
• Updates existing records by preserving their first seen timestamp, refreshing last seen, and incrementing seen count.
• Adds --fs / --first-seen to print records first observed in the current run.
• Adds --ls / --last-seen N to print missing records last seen at least N days ago.
• Adds --lsd / --last-seen-date to print missing records last seen before a US date/datetime.
• Reads old JSON files that do not yet contain the new metadata, including dashed and snake_case legacy keys.
• Documents the DNS history metadata and flags in the README.
• Adds an isolated CMake/CTest regression test for DNS history metadata that does not require network, Boost, or OpenSSL.

Verification:

• git diff --check passes.
• Configured the isolated history test with CMake using -DCMAKE_POLICY_VERSION_MINIMUM=3.5 -DDOOKED_BUILD_HISTORY_TESTS=ON.
• Built dooked_dns_history_test with MSVC.
• ctest --output-on-failure passes for the DNS history regression test.
• Full app build still requires configured Boost/OpenSSL development dependencies in my local environment; the history regression target is isolated so reviewers can verify the new metadata behavior without DNS/network/SSL setup.

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