feat(build): add mix burrito.dinein for local binary builds (#112)

## Summary

- Adds `mix burrito.dinein` — a repo-level Mix task that auto-detects
the current host platform and builds the Burrito-wrapped `lc` release
for that single target only
- Renames the output from `app/burrito_out/lc_<target>` to
`app/burrito_out/lc` so it's immediately runnable without knowing the
target suffix
- Fails with an actionable error on unsupported platforms (macOS Intel,
Linux ARM, etc.), listing the three supported targets

## What it does

```
mix burrito.dinein                        # builds for current host
mix burrito.dinein --target linux_x86_64  # explicit override
```

Runs `MIX_ENV=prod BURRITO_TARGET=<detected> mix release lc --overwrite`
inside `app/`, mirroring the pattern from `mix container.build`. After a
successful build, renames the target-suffixed binary to plain `lc` (or
`lc.exe` on Windows) in `app/burrito_out/`.

## Test plan

- [x] `mix compile --warnings-as-errors` passes
- [x] `mix test` passes — 8 tests (6 existing + 2 new)
  - Rejects extra positional arguments with usage message
  - `detect_target!/0` returns a known target name for the current host

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bougyman

lib/mix/tasks/burrito.dinein.ex

@@ -0,0 +1,122 @@
+defmodule Mix.Tasks.Burrito.Dinein do
+  @shortdoc "Builds a local Burrito lc binary for the current host platform"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+      mix burrito.dinein [--target TARGET]
+
+  Detects the current host platform, builds `app/`'s Burrito binary for that
+  single target only (`MIX_ENV=prod BURRITO_TARGET=<target> mix release lc
+  --overwrite`), then installs it as `app/burrito_out/lc` (dropping the
+  target suffix) so it's immediately runnable as `./app/burrito_out/lc`.
+
+  Supported targets (auto-detected from the current host):
+  - `macos_aarch64` — macOS Apple Silicon
+  - `linux_x86_64` — Linux x86_64
+  - `windows_x86_64` — Windows x86_64
+
+  Pass `--target TARGET` (`-t`) to override detection — useful for explicit
+  cross-compilation when Zig can produce the target from this host.
+  """
+
+  use Mix.Task
+
+  alias RepoTasks.Shell
+
+  @supported_targets ~w[macos_aarch64 linux_x86_64 windows_x86_64]
+
+  @impl Mix.Task
+  def run(argv) do
+    {opts, args} =
+      OptionParser.parse!(argv, strict: [target: :string], aliases: [t: :target])
+
+    unless args == [] do
+      Mix.raise("Usage: mix burrito.dinein [--target TARGET]")
+    end
+
+    target = opts[:target] || detect_target!()
+
+    Mix.shell().info("==> Fetching app/ deps")
+    Shell.run!("mix", ["deps.get"], cd: "app")
+
+    Mix.shell().info("==> Building Burrito binary for #{target}")
+
+    Shell.run!(
+      "mix",
+      ["release", "lc", "--overwrite"],
+      cd: "app",
+      env: [{"MIX_ENV", "prod"}, {"BURRITO_TARGET", target}]
+    )
+
+    # Burrito names output lc_<target> (lc_<target>.exe on Windows).
+    # Rename to plain lc (lc.exe) so it's immediately usable without knowing
+    # the target suffix.
+    ext = if target == "windows_x86_64", do: ".exe", else: ""
+    src = Path.join("app/burrito_out", "lc_#{target}#{ext}")
+    dst = Path.join("app/burrito_out", "lc#{ext}")
+
+    case File.rename(src, dst) do
+      :ok ->
+        Mix.shell().info("==> Built: #{dst}")
+
+      {:error, reason} ->
+        Mix.raise("Failed to install #{src} as #{dst}: #{:file.format_error(reason)}")
+    end
+
+    :ok
+  end
+
+  @doc """
+  Returns the Burrito target name matching the current host platform.
+
+  Raises `Mix.Error` with an actionable message on unsupported platforms.
+  """
+  @spec detect_target!() :: String.t()
+  def detect_target! do
+    os = detect_os()
+    cpu = detect_cpu()
+
+    case {os, cpu} do
+      {:darwin, :aarch64} ->
+        "macos_aarch64"
+
+      {:linux, :x86_64} ->
+        "linux_x86_64"
+
+      {:windows, :x86_64} ->
+        "windows_x86_64"
+
+      {os, cpu} ->
+        Mix.raise(
+          "Unsupported platform: #{os}/#{cpu}. " <>
+            "Supported targets: #{Enum.join(@supported_targets, ", ")}"
+        )
+    end
+  end
+
+  defp detect_os do
+    case :os.type() do
+      {:win32, _} -> :windows
+      {:unix, :darwin} -> :darwin
+      {:unix, :linux} -> :linux
+      {:unix, other} -> other
+    end
+  end
+
+  defp detect_cpu do
+    arch =
+      :erlang.system_info(:system_architecture)
+      |> to_string()
+      |> String.downcase()
+      |> String.split("-")
+      |> List.first()
+
+    case arch do
+      "x86_64" -> :x86_64
+      "aarch64" -> :aarch64
+      "arm64" -> :aarch64
+      other -> other
+    end
+  end
+end

test/mix/tasks/burrito.dinein_test.exs

@@ -0,0 +1,16 @@
+defmodule Mix.Tasks.Burrito.DineinTest do
+  use ExUnit.Case, async: true
+
+  alias Mix.Tasks.Burrito.Dinein
+
+  test "rejects extra positional arguments" do
+    assert_raise Mix.Error, "Usage: mix burrito.dinein [--target TARGET]", fn ->
+      Dinein.run(["extra"])
+    end
+  end
+
+  test "detect_target! returns a known supported target for the current host" do
+    target = Dinein.detect_target!()
+    assert target in ~w[macos_aarch64 linux_x86_64 windows_x86_64]
+  end
+end