🟡 Port problem 12 (Integer to Roman) from Clojure/Swift to C++23

[AF2B]

Summary

• Converts clojure/12.clj and swift/12.swift (Integer to Roman, solved twice) to C++23 at solutions/0001-0100/0012-integer-to-roman/.
• Both old solutions used the identical greedy symbol-table algorithm, so this ports one approach rather than two redundant copies of the same logic.
• Removes both now-converted old files.

Base is chore/cpp23-migration (#4) — retarget to main once that lands.

Test plan

• CI (build & test, static analysis, format check) passes.