Migrate IntaRNA to strict C++23 - #234

Implements phase 2 of discussion #232.

• requires strict C++23 through the pinned official Autoconf Archive probe
• keeps predictor and energy recurrences unchanged
• updates the test-only Catch header and explicit standard includes
• validates GCC 14 release/debug and Apple Clang/libomp release in CI
• compiles all installed public headers and an external pkg-config consumer
• documents scientific invariants plus the benchmark-gated phase 3/4 roadmap

Final gate: 4,275 API assertions in 36 cases and all 20 CLI cases passed with explicit -std=c++23; all 65 public headers compile independently. Required GCC 14 and Apple Clang evidence is green in this PR.

Phase 1 scientific corrections are in #233.