Adds the parallel Rust workspace under rust/ per REWRITE.md:
- ewm-core (headless lib, zero dependencies) and ewm (binary,
declares sdl2 now so CI installs SDL2 headers from day one;
prints the same usage line as the C binary)
- rust-toolchain.toml pinning 1.91.1 with rustfmt + clippy
- GitHub Actions CI: C build job (ubuntu + libsdl2-dev) and Rust
job (fmt --check, clippy -D warnings, test)
- Anchor the 'ewm' gitignore pattern to src/ so rust/ewm is tracked;
ignore rust/target/
- README note pointing at REWRITE.md
The C tree in src/ is untouched.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
