fix: make rustler dep unconditionally optional for Hex

Rustler should always be optional: true so Hex shows it as (optional).
End users get precompiled NIFs via rustler_precompiled; they don't need
Rust installed. Dev/test force_build is now in native.ex via Mix.env().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cigrainger

lib/dux/native.ex

@@ -17,7 +17,9 @@ defmodule Dux.Native do
       x86_64-pc-windows-msvc
     ),
     nif_versions: ["2.16"],
-    force_build: System.get_env("DUX_BUILD") in ["1", "true"]
+    force_build:
+      System.get_env("DUX_BUILD") in ["1", "true"] or
+        Mix.env() in [:dev, :test]
 
   # Database lifecycle
   def db_open, do: :erlang.nif_error(:nif_not_loaded)

mix.exs

@@ -4,8 +4,6 @@ defmodule Dux.MixProject do
   @version "0.1.0"
   @source_url "https://github.com/elixir-dux/dux"
 
-  @force_build? System.get_env("DUX_BUILD") in ["1", "true"]
-  @dev? Mix.env() in [:dev, :test]
 
   def project do
     [
@@ -38,7 +36,7 @@ defmodule Dux.MixProject do
   defp deps do
     [
       {:rustler_precompiled, "~> 0.8"},
-      {:rustler, "~> 0.37.3", optional: not (@dev? or @force_build?)},
+      {:rustler, "~> 0.37.3", optional: true},
       {:nx, "~> 0.9", optional: true},
       {:benchee, "~> 1.3", only: [:dev, :test]},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},