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.

xtask/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[[bin]]
+name = "xtask"
+path = "src/main.rs"
+
+[dependencies]
+# CLI argument parsing
+clap = { version = "4.4", features = ["derive"] }
+# Error handling
+anyhow = "1.0"
+# Colors
+colored = "2.1"
+# Command existence check
+which = "5.0"