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.

src/commands/doctor.rs

@@ -1019,6 +1019,7 @@ fn is_freebsd() -> bool {
     cfg!(target_os = "freebsd")
 }
 
+#[cfg(unix)]
 fn serial_group_name() -> &'static str {
     if is_freebsd() {
         "dialer"
@@ -1027,6 +1028,7 @@ fn serial_group_name() -> &'static str {
     }
 }
 
+#[cfg(unix)]
 fn serial_group_fix_command() -> String {
     if is_freebsd() {
         "sudo pw groupmod dialer -m $USER".to_string()
@@ -1071,6 +1073,8 @@ fn check_operator_group() -> bool {
     }
 }
 
+#[cfg(unix)]
+#[allow(dead_code)]
 fn whoami() -> String {
     std::process::Command::new("whoami")
         .output()