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.

README.md

@@ -377,65 +377,18 @@ Anvil is written in Rust and compiles to a single static binary. You need:
 
 - **Rust toolchain** (stable, 2021 edition or later)
 - **A C linker** (`gcc` or equivalent -- Rust uses it under the hood)
-- **MinGW** (only if cross-compiling a Windows binary from Linux)
-- **zip** (only for packaging release artifacts)
 
-### Linux / WSL from scratch
-
-A fresh Ubuntu or WSL instance needs three commands:
+### Build
 
 ```bash
-# 1. System packages (C linker + cross-compile + packaging tools)
-sudo apt update && sudo apt install build-essential mingw-w64 zip
-
-# 2. Rust toolchain
-curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
-source ~/.cargo/env
-
-# 3. Build
 cargo build --release
 ```
 
-The release binary lands at `target/release/anvil`. Copy it somewhere in
-your PATH.
-
-### Windows (native)
-
-Install Rust from [rustup.rs](https://rustup.rs), which includes the MSVC
-toolchain. Then:
-
-```
-cargo build --release
-```
-
-The binary lands at `target\release\anvil.exe`.
-
-### Release builds (Linux + Windows from one machine)
-
-The `build-release.sh` script at the repo root builds optimized, stripped
-binaries for both platforms and packages them into tarballs and zips. It
-reads the version from `Cargo.toml` (the single source of truth) and
-accepts an optional suffix for pre-release builds:
-
-```bash
-./build-release.sh              # uses version from Cargo.toml (e.g. 1.0.0)
-./build-release.sh beta1        # appends suffix (e.g. 1.0.0-beta1)
-./build-release.sh rc1          # appends suffix (e.g. 1.0.0-rc1)
-```
-
-This produces a `release-artifacts/` directory with:
-
-```
-anvil-1.0.0-linux-x86_64.tar.gz
-anvil-1.0.0-linux-x86_64.zip
-anvil-1.0.0-windows-x86_64.zip
-SHA256SUMS
-```
-
-Upload these to a Gitea release. The script requires `build-essential`,
-`mingw-w64`, and `zip` as described above.
+The release binary lands at `target/release/anvil` (or `target\release\anvil.exe`
+on Windows). Copy it somewhere in your PATH.
 
 ### Running the test suite
+
 ```bash
 cargo test
 ```
@@ -449,6 +402,7 @@ skip gracefully and everything else still passes.
 #### Full test suite on Linux / WSL
 
 The e2e tests need `cmake`, `g++`, and `arduino-cli` with the AVR core:
+
 ```bash
 sudo apt install cmake g++
 curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
@@ -460,6 +414,7 @@ arduino-cli core install arduino:avr
 
 Install [arduino-cli](https://arduino.github.io/arduino-cli/installation/)
 and add it to your PATH, then install the AVR core:
+
 ```
 arduino-cli core install arduino:avr
 ```
@@ -470,6 +425,61 @@ a Visual Studio Developer Command Prompt (which provides `cl.exe`).
 
 ---
 
+## Building Release Binaries
+
+Anvil uses a Rust `xtask` workspace member to build release binaries for all
+platforms from a single machine. No VMs, no containers, no root required.
+
+### How it works
+
+The xtask build system uses [cargo-zigbuild](https://github.com/rust-cross/cargo-zigbuild)
+with the [Zig](https://ziglang.org) compiler as a universal cross-linker.
+From a single FreeBSD machine you get all three platform binaries:
+
+```
+release-artifacts/
+    anvil-X.Y.Z-freebsd-x86_64.tar.gz    native FreeBSD build
+    anvil-X.Y.Z-linux-x86_64.tar.gz      cross-compiled via zig
+    anvil-X.Y.Z-windows-x86_64.zip       cross-compiled via zig
+    SHA256SUMS
+```
+
+### First-time setup
+
+```bash
+cargo xtask --fix
+```
+
+This installs zig (via pkg), zip, cargo-zigbuild, and the required rustup
+cross-compile targets. Only the zig and zip installation steps require sudo.
+
+### Building a release
+
+```bash
+cargo xtask                      # build all platforms, version from Cargo.toml
+cargo xtask --suffix rc1         # pre-release (e.g. 1.0.0-rc1)
+```
+
+### Other commands
+
+```bash
+cargo xtask --check              # verify all dependencies are installed
+cargo xtask --clean              # remove cross-compile artifacts
+```
+
+### Platform support
+
+| Host | FreeBSD binary | Linux binary | Windows binary |
+|---|---|---|---|
+| FreeBSD | native | cargo-zigbuild | cargo-zigbuild |
+| Linux | not possible | native | cargo-zigbuild |
+| Windows | not possible | not tested | native |
+
+FreeBSD is the recommended build host -- it produces all three binaries in
+a single run.
+
+---
+
 ## License
 
 MIT -- see [LICENSE](LICENSE).