0.1.2

Release 0.1.2

- Add --tip-file option for explicit tip names and dates
- Refactor date utilities into core/dates.h/.cpp
- Wrap main() in try/catch for clean error handling
- Add GitHub Actions CI (Linux x86_64, macOS arm64)
- Remove -ffast-math, guard -msse3 for x86_64 only
- Fix Tree_annotation<auto> for Clang C++20 compatibility
- Add CLAUDE.md and CONTRIBUTING.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>