feat(xtask): cross-platform release build via cargo-zigbuild

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.

Cargo.toml

@@ -1,3 +1,7 @@
+[workspace]
+members = [".", "xtask"]
+default-members = ["."]
+
 [package]
 name = "anvil"
 version = "1.0.0"