GitHub Actions CI pipeline

[tale]

Set up GitHub Actions for continuous integration:

• Build — cargo build on macOS and Linux
• Test — cargo test (once Integration test suite #17 adds tests)
• Lint — cargo clippy -- -D warnings
• Format — cargo fmt --check
• Lua lint — validate Lua stdlib files (e.g. luau-analyze or basic syntax check)
• Docs — build the Vitepress docs site to catch broken links/includes

Optionally, a release workflow that builds binaries for macOS (aarch64 + x86_64) and Linux (x86_64) and attaches them to GitHub releases.