Build & attach macOS prebuilt binaries to v0.2.0-alpha.1

[Fanaperana]

Per the README install matrix, we're missing macOS tarballs for v0.2.0-alpha.1.

Targets needed:

• x86_64-apple-darwin (Intel Mac)
• aarch64-apple-darwin (Apple Silicon)

Recipe (run on the matching host):

git checkout v0.2.0-alpha.1
cargo build --release -p spdf-cli
VER=0.2.0-alpha.1
TARGET=$(rustc -vV | awk '/^host:/ {print $2}')
DIR="spdf-${VER}-${TARGET}"
mkdir -p "dist/${DIR}"
cp target/release/spdf "dist/${DIR}/"
cp LICENSE README.md CHANGELOG.md "dist/${DIR}/"
tar czf "dist/${DIR}.tar.gz" -C dist "${DIR}"
shasum -a 256 "dist/${DIR}.tar.gz" > "dist/${DIR}.tar.gz.sha256"
gh release upload v${VER} "dist/${DIR}.tar.gz" "dist/${DIR}.tar.gz.sha256"

After uploading, check the target off in the release notes body and the README matrix.