Replace build-all.sh shell script with a Rust xtask workspace member.
Uses cargo-zigbuild + zig as a universal cross-linker -- no VMs, no
containers, no root required.
Produces all three release binaries from a single FreeBSD machine:
anvil-X.Y.Z-freebsd-x86_64.tar.gz (native cargo build)
anvil-X.Y.Z-linux-x86_64.tar.gz (cargo zigbuild)
anvil-X.Y.Z-windows-x86_64.zip (cargo zigbuild)
Commands:
cargo xtask --fix install zig, zip, cargo-zigbuild, rustup targets
cargo xtask --check verify all dependencies
cargo xtask build all three binaries + SHA256SUMS
cargo xtask --clean remove cross-compile artifacts
cargo xtask --suffix rc1 build with version suffix
Also converts Cargo.toml to a workspace (members: anvil, xtask).
build-all.sh retained as a thin wrapper around cargo xtask.
